Operation Downfall 3: Allied Intelligence

Allied Intelligence and Japanese Preparations

As the planning for the invasion of Japan picked up momentum, US intelligence estimated that the combat forces holding Kyushu in mid-1944 consisted of one combat division and two depot divisions. A combat division was the principle all-arms ground combat unit of the Japanese Army with a troop complement of 16,000 men while depot divisions were essentially pre-stocked caches of weapons, equipment and supplies with a cadre of personnel and training staff that were used to generate replacements and create new combat divisions. The US War Department's Military Intelligence Service believed that over half the Japanese personnel on the island were Navy ground troops and support personnel from the Army and Naval Air Arm, with the Japanese 16th Area Army Headquarters having overall command. The Joint Intelligence Committee (who served the JWPC) believed that by the time of the invasion, the Japanese would have augmented their forces to around six combat divisions and two depot divisions and after the invasion started would have reinforced their garrison with another four divisions as ten divisions was probably the most they could support given the geography and infrastructure of the island.

Downfall, Operation: - landing beaches

Downfall, Operation: - allied assault force

Downfall, Operation: Map 7b: Beach organisation for Operation Olympic

Knowledge of the strength and disposition of Japanese forces on Kyushu relied on intercepted communications as there were no effective agents or spy networks on the home islands or western sympathisers who had access to that sort of information in the sort of detail required. Aerial reconnaissance played an important role in detecting force movements and identifying physical targets such as airbases, depots, factories, concentrations of weapons and vehicles. Its effectiveness was constrained by weather, darkness and technology. PoW interrogations had provided an important amount of information but there were few prisoners available in the Spring of 1945 that could shed any light on the defence preparations on the Home Islands. US intelligence had been intercepting and decrypting Japanese diplomatic communications since well before the Second World War with intelligence being gathered on a regular basis, except for short periods after the Japanese had changed their encryption system. The Americans were able to decipher Japanese Naval communications relatively soon after Pearl Harbor but it wasn't until April 1943 that they had managed to decipher Japanese Army communications and it wasn't until the beginning of 1944 that sizable amounts of data were available to the US Intelligence Staffs. Just to note, codes and ciphers are different things. Saying 'the Eagle has landed' to denote that an action has just taken place can be called a code. If that phrase is then turned into numbers and then scrambled according to a system that requires a key to unscramble, then it can be considered encrypted. To take advantage of the substantial amount of information in the Japanese Army codes, required both deciphering and decoding and then the resulting information provided to military force analysts to interpret.

Up until mid-1944, the Imperial General Staff had given little attention to the direct defence of the Japanese Home Islands, perceiving that there were only minor threats from small-scale American air raids carried out to weaken Japanese morale, strengthen American morale, or divert attention away from up-and-coming operations. Therefore, the early emphasis on homeland defence concentrated on airpower, however the few available anti- aircraft guns and fighter interceptors were obsolescent and the organisation of these defences were hampered by poor training and inadequate equipment with the air warning system relying mainly on visual observation and picket boats. However, on 18 April 1942, Lt Col James Doolittle led a force of medium bombers off of the carrier Hornet in a raid on Tokyo. The raid highlighted the inadequate nature of the Japanese air defence system and so the Imperial General Headquarters ordered modern planes, guns, equipment and radar. Air defence forces were removed from the control of ground-based commanders and organised into an air army and placed directly under a Commander of Homeland Defence. Despite this, an actual invasion of the Home Islands was still considered almost impossible and air defence was the primary focus. What happened in mid-1944, was that the war entered a dangerous new phase with Nimitz's Central Pacific forces landing on Saipan on 15 June 1944, bringing US forces to within 1500 miles of Tokyo, MacArthur's forces in the Southwest Pacific area were ready to attack the Philippines and B-29s from China bombed industrial targets in southern Kyushu. The Commander of Homeland Defence was tasked with producing a new comprehensive plan that included measures to defend the Japanese coast near strategically important targets. The crippling losses in the Battle for the Philippine Sea forced a complete re-evaluation of Pacific strategy. An invasion of the Home islands ceased to become a possibility and became a probability. Japanese strategists concluded that the USA would seek air and naval bases nearer Japan before launching a final assault. In looking at likely avenues of approach, the Japanese concluded that an advance from the north, via the Kurils and Aleutians was unlikely and that the main threat would come from the south and east, with the fall of the Marianas and the imminent invasion of the Philippines.

As a result of these new threats, the Imperial General Headquarters issued a new directive in relation to Japanese strategy in the Pacific. This was called 'Sho-Go' (Victory Operation) and focused on delaying the American advance while causing the maximum attrition of US forces in the Philippines, Taiwan and the Ryukyus with two objectives in mind - to inflict such losses and delays that it would persuade Allied leaders to opt for a negotiated peace or if that failed, to buy time to prepare the defence of the Home Islands. The first and second parts of the Sho-Go plan covered the defence of the Philippines, Formosa and the Ryukyus, while part three covered the Home Islands of Shikoku, Honshu and Kyushu. A number of the concepts found in the Sho-Go operation, featured in the final 1945 plan for the defence of the Home Islands, Ketsu-Go (Decisive Operation). Sho-Go emphasised having a strong defence on the beach and launching vigorous counterattacks against the landing forces before they could establish a secured beachhead. It called for self-contained battalion- sized fighting positions and gun emplacements and took the decision not to withdraw troops from overseas garrisons but to fight using standing homeland defence troops, garrison forces and newly raised troops, strengthened with some veterans from the Kwantung Army in Manchuria.

Construction of the Home Island defences started in the autumn of 1944 but by early 1945, little had been accomplished except in four areas - Tokyo Bay, Ise Bay (Nagoya), Shikoku and southern Kyushu. Construction on the Izu islands (near the mouth of Tokyo Bay) and in southern Kyushu progressed more rapidly than elsewhere but overall, progress was behind schedule as Japanese officers, who were trained almost exclusively in offensive tactics found it difficult to plan for a defensive campaign and two vital materials, steel and concrete, were in short supply. Such fortifications had to be hidden, from both the enemy and the Japanese populace, so as not to spread defeatism. By the middle of April 1945, the situation had changed dramatically for the Japanese and events had rendered the Sho-Go plan obsolete. The Philippines had just fallen (and with it the destruction of General Yamashita Tomoyuki's Fourteenth Area Army) and while the battle for Okinawa had just begun, it would go on for another ten weeks. The futility of continued Japanese resistance was not only embodied in the frantic kamikaze attacks that were taking place against the US invasion fleet but the destruction of the battleship Yamato and its cruiser escort. The world's largest battleship had sorted from the Inland Sea on 6 April 1945 to attack the US Fleet and maintain communications with Lt Gen Ushijima Mitsuru's Thirty-Second Army on Okinawa. Even though the battle was just beginning, the result was obvious to all except the most diehard Japanese. The Japanese now faced the very real likelihood that they would have to defend against a direct Allied attack against the Home Islands.

In the weeks following the despatch of the planning directive to both MacArthur and Nimitz on 3 April 1945, intercepted communications were already starting to show that the Japanese were already considering an attack on the Home Islands as a distinct possibility. A message sent by the German Naval Attache in Japan, described a report that outlined the Japanese preparations for an invasion of their homeland and that they expected an invasion of Okinawa shortly and ultimately, an invasion of the Tokyo Plain. Throughout 1945, the Japanese tried to anticipate American moves based on previous experience of US operations or on what they believed the Americans to be planning. Neither method required up-to-date intelligence data, important as by the spring of 1945 the Japanese had largely lost the air and sea capability to observe the forces of the USA. So as American planning staff debated as to what they might do the autumn of 1945, the Japanese were pondering what they were going to do as well. The Japanese tried to estimate American attentions by simulating their decision-making process. The Japanese reached roughly the same conclusions as their American counterparts. In the 'Report to the Throne' from the Imperial General Headquarters dated 19 January 1945, Japanese planning staff that it was likely that the US would conduct a two-pronged advance from the Mariana Islands towards the Iwo Jima island group and from the Philippines towards either Taiwan, Shanghai or Okinawa, with the two prongs of the assault converging on the Home Islands sometime in late 1945. The very next day, 20 January 1945, the Imperial General Headquarters issued directives entitled an 'Outline of Army and Navy Operations' that stated that strongpoints were to be constructed on Iwo Jima, Formosa (now Taiwan), Okinawa, the Shanghai district and the south Korean coast, indicating that such preparations should be complete by the early autumn of 1945. The Imperial General Headquarters issued an 'Outline of Preparations for the Ketsu-Go Operation' on 8 April 1945.

By the spring of 1945, Japanese predictions of American intentions were quite accurate. They had concluded that an invasion of the Home Islands was inevitable and the only question was over the number of supporting operations between the end of the battle for Okinawa and the invasion. A the battle for Okinawa raged, many Japanese commanders predicted that the next American target would be Kyushu and some even correctly guessed as to the likely plan for the operation, that US forces would land in southern Kyushu to seize additional air and naval bases for an eventual operation against the Tokyo Plain. This was not a universal view however and some officers feared a strike against lightly defended Shikoku or that northern Kyushu might be the target. If an invasion of southern Kyushu had not taken place by the middle of the autumn of 1945 or if preliminary operations had been undertaken against Cheju and the Tsushima Islands, then the threat to northern Kyushu would have been upgraded. The greatest fear would be that the US would attack Kyushu straight after the battle for Okinawa had ended when the fortifications were nowhere near complete. As spring moved into summer, intercepted communications indicted that the Japanese had begun mining harbours and evacuating civilians from areas of likely combat. The central question remained however, would the United States conduct additional peripheral operations against southern China, Korea or perhaps Oshima to support an attack on Kyushu in the autumn or strike directly at the Home islands? Army analysts tended to think the former, Navy analysts believed the latter. As this was being debated, additional troop movements from the Asian mainland were taking place to move a combat division from Manchuria to Kyushu as well as troop transfers from the Kurils. An assessment by MacArthur's Intelligence Staff indicated that the Japanese were now moving towards preparing for an all-out defence of their Home Islands, with new combat formations being created and the flow of troop replacements to the outlying garrisons having stopped, while troops guarding the immediate approaches to Japan (such as on Iwo Jima and Okinawa) were being sacrificed in desperate delaying actions. By early May 1945, US Intelligence estimated that the Japanese has some 246,000 military personnel on Kyushu of which about 128,000 were in Army ground combat units. The Intelligence Staff estimated that the four additional divisions expected by 1 November 1945 and the increase in support troops would add another 100,000 men to the total on the island.

The collapse of Germany in early May forced the Japanese to alter their estimates as to the future course of Allied strategy. They reasoned that the collapse of the Third Reich would mean that public opinion in the United states would not tolerate a long and costly campaign to finish Japan off and so a great deal of political pressure would accumulate on the military to finish off the Pacific War as quickly as possible and with as few losses as possible. The Japanese however, wildly overestimated the speed with which the Americans would be able to transfer forces from the European Theatre over to the Pacific. The Japanese estimated that around thirty divisions would be transferred, with an initial ten arriving in the Philippines by the end of July and the other twenty to arrive by the end of August. Their assessments led to the Japanese to conclude that the Americans were unlikely to conduct additional peripheral operations and instead launch a direct assault on the Home Islands. As May turned to June, the Japanese refined this assessment further and concluded that southern Kyushu would be a likely target and that the United States would attack with a force of between ten and twelve divisions (which was later revised upwards) at some point after 1st October 1945 in order to acquire additional air and naval bases to support a thirty division invasion of the Kanto Plain around three months after the Kyushu landings. Assessments of the terrain and layout of Kyushu as well as studies of previous American operations led the Japanese towards the same conclusions already reached by the US planning staff. The main objectives would consist of a combination of the large airfields at Kanoya and Miyakonojo, as well as the harbours and port facilities at the two large bays in southern Kyushu, Kagoshima Bay and Ariake (Shibushi). The Japanese estimated that the Americans would land to secure these objectives on three separate beaches - Ariake, Miyazaki and Kushikino. They initially believed that the main US effort would be at Miyazaki but eventually came to conclude that they would have sufficient forces (fifteen divisions) to land at all three beaches equally. The landings would be preceded by a landing on the island of Tanega to secure facilities for small naval craft and patrol aircraft. The Japanese then expected simultaneous landings by five or six divisions at Ariake bay, four divisions near Miyazaki, two divisions near Kushikino with another two-division force landing on southern Shikoku and support from one or two airborne divisions landing to secure airfields within the interior of Kyushu.

By early June, further intercepted communications had revealed that the Japanese had moved two more divisions to Kyushu, one from Hokkaido and another from Korea, while activating two Army-level headquarters in north and south Kyushu. As the Japanese tended to subordinate three divisions to an army-level headquarters, it tended to confirm that the Japanese were aiming to deploy six combat divisions on Kyushu, evenly split between the two parts of the island. Added to this, the Japanese seemed to be preparing to conduct wide-ranging suicide attacks with one series of messages indicating that up to 2,000 obsolete planes were being used to equip and train Kamikaze units, in some cases for night operations. Other messages indicated the construction of extensive underground facilities, including hangers and new dispersal airfields on Kyushu. A message intercepted in mid-June contained a description from a Japanese naval base commander about the progress being made on the construction of suicide boats with another message revealing the presence of a base for piloted suicide torpedoes (kaiten) on the south-eastern tip of Kyushu. Even though the Japanese had correctly guessed American intentions they faced a dilemma, as all defenders against amphibious assaults do, of whether to defend the beach strongly in order to repulse the enemy back into the sea, or just enough to delay them and keep a mobile reserve further inland away from the enemy's air and naval power and then hit them with a counterattack at an opportune moment. Both strategies held risks. To concentrate on beach defence meant guessing the correct landing sites that the enemy were going to use and the strength they were going to attack with. If the enemy broke through the defences then the battle would be over. Holding reserves away from the beach meant that the defenders had more flexibility and could see the enemy plan developing but would necessitate bringing together forces scattered over a wide area, concentrating them and then launching a coordinated attack, something that would not be easy, especially when the enemy held air superiority, as the Germans had found out in Normandy.

As additional information reached the Americans that Naval personnel were being assigned tasks traditionally reserved for Army troops, including the operation of antiaircraft gun sites and static defence of military facilities, the Japanese issued guidelines as to how they would defend the beaches identified as possible landing sites. All the commanders were to prepare fortifications and to deploy their troops to defend the Home Islands 'at the water's edge' as they concluded that US forces could not be allowed to establish a beachhead. Static coastal defence divisions were to hold the Americans on the beaches and contain them there, being organised to man fixed fortifications on the beach. Mobile assault divisions were to be held behind the beaches in order to counterattack the US forces within ten days. By engaging in close assaults and fighting right up next to the American forces, the Japanese might nullify the US advantage in naval and air power for fear of hitting friendly forces.

Almost until the end of the war, American military planners feared that the Japanese would transfer portions of the crack Kwantung Army from Manchuria in order to help defend the Home Islands. Beyond transferring a few units, the Japanese had no such plans to bring home the army, partly because the Kwantung Army had been denuded of units already to fight in the Pacific and so its effectiveness had declined considerably and the Japanese considered that both Manchuria and Korea would both be important to support the battle for the Home islands and feared a Soviet attack on Manchuria. The Japanese planned to defend the Home Islands with a newly raised army consisting of fresh recruits bolstered by garrison troops and a few veteran units from elsewhere in the Empire. They planned to raise a new force of some forty divisions, twenty independent brigades as well as the requisite support units, in total some two million men. As already mentioned the new units would consist of two types, static coastal defence divisions and better trained and equipped manoeuvre divisions to serve in counterattacks against the American beachheads. The former would be less mobile but have more heavy artillery. The Japanese would conduct three separate call-ups between March and July 1945 so as not to overstretch their remaining logistics. However, as the battle for Okinawa began to wind down, the Japanese started to worry that the initial invasion might come in June and so forces were moved from the Kurils to Honshu and Hokkaido to Kyushu in case the Americans moved early and so the defence of Hokkaido was downgraded to a holding action only. The Japanese also mobilised their new forces slightly quicker than they had anticipated and so put a strain on the logistics system with transportation bottlenecks developing, scarcities in equipment and supplies appeared and shortages in trained and experienced officers occurred that disrupted the creation of unit headquarters. The first call up in March was dedicated to creating eighteen static coastal defence divisions, thirteen dedicated to the defence of Honshu, Shikoku and Kyushu. The second wave, during May, created eight new manoeuvre divisions, six tank brigades and five tank regiments. At the same time, four veteran divisions arrived from Manchuria. The third mobilization came in July, creating sixteen new divisions with nine slated as being static coastal defence and seven for manoeuvre. The Japanese also managed to create fourteen independent brigades, five infantry regiments and a large number of artillery units in this last mobilisation.

The Japanese seemed to be able to field new forces very quickly and from afar they seemed quite formidable. The US War Department's Military Intelligence Service revised its estimates of enemy strength on Kyushu in mid-June to around 300,000 that occurred just before President Truman met with the Joint Chiefs of Staff to discuss the plans for the invasion of Japan. The developments in the Japanese forces defending Kyushu were still consistent with what had been projected a year earlier regarding an invasion on 1 November 1945. As outlined previously, Truman met with the Joint Chiefs of Staff on 18 June to go through the plan to end the Pacific War and preparations for the Potsdam Conference. A few days before, his Chief of Staff, Admiral Leahy, sent a memo ('Memorandum for the Joint Chiefs of Staff' from Admiral Leahy, enclosure to JPS 697/D, 14 June 1945) to the JPS and JWPC outlining that the President would make his decision as to the course of action to be taken against Japan on the basis of the estimate casualties for the various options available. This seemed to catch a number of staff officers off guard. Shortly afterwards, a telephone conversation occurred between the senior US Navy representative, Admiral Donald Duncan, and his Army counterpart, General George A Lincoln. Duncan was worried that there was in fact another decision coming up, given that there had already been a specific directive (JCS 1331/3, 'Directive for Operation Olympic', 25 May 1945 that amongst other things, assigned the command of the operation to MacArthur and set the start date as 1 November 1945) setting in motion preparations and planning to conduct Operation Olympic, the first part of Operation Downfall. Duncan pointed out that the commitment had probably been made without specifically being reviewed and cleared by the President, although he mentioned that Roosevelt and Churchill (during their meeting in Quebec) had approved the overall objective the directive supported. Discussions rumbled on for the next few days, while the JWPC submitted its draft of the requested paper (JWPC 369/1, 15 June 1945, 'Details of the campaign against Japan') to the JPS. It basically presented the same case for an invasion of Kyushu with the same forecast of Japanese forces that had been present in estimates since mid-1944. In response to the request for casualty estimates, the JWPC offered these figures, followed by a highlighting of the uncertainty surrounding them as the degree of opposition encountered and time required to complete the campaign were both major variables that could affect the figures:

While the JPWC assessment did not give a specific breakdown for the casualty figures for each area within the scenarios, a breakdown can be established by comparing the component figures between each scenario. For example, the difference between the second and third scenarios (for total casualties) is 87,500, the difference between the two being the inclusion of an attack on the Tokyo Plain. Therefore, assuming that 87,500 casualties have been estimated for the assault on the Tokyo Plain, then the casualty estimates for the attack on southern Kyushu is 106,000 (193,500 - 87,500) and northern Kyushu as 26,500 (220,000 - 193,500). These figures were scenario driven however, and an attack against northern Kyushu without assaulting the south of the island first would have generated a different set of figures. The next day, a revised set of figures were circulated to the JCS through the JPS, which had made a few minor alterations, the two biggest were that they had deleted the casualty estimate table and the figure for the total number of US military personnel involved in the Kyushu operation (JCS 1388, 16 June 1945, 'Details of the Campaign Against Japan'). In order to compromise between on the one hand, not showing any casualty figures at all and on the other, finding a set of figures that were acceptable to the JPS, the Army's Director of Operations, Major General J E Hull, asked his staff for the casualty figures from the campaigns on Okinawa, Iwo Jima, Luzon and Leyte, and the overall figures for MacArthur's campaigns to date. This information, he stated, would be used as background material for the up and coming meeting with the President, who was 'very much disturbed over losses on Okinawa'. Hull obtained the following figures:

In comparison, the first thirty days of the Normandy Campaign, the US suffered about 42,000 casualties. The figures for MacArthur's campaign overall (March 1944 - May 1945) were 13,742 US personnel killed and 310,165 Japanese personnel killed, a 1:22 ratio. The Okinawa Campaign was ongoing at the time and the figures were updated on 11 July 1945 in time for the Potsdam Conference, when US casualties had risen to about 47,000 and Japanese about 119,000. At about the same time as Hull was pulling together these estimates, Marshall asked MacArthur for the casualty estimates that he was using for planning purposes with regard to the first ninety days of Olympic. The response was a figure of 50,800 for the first thirty days of the operation and a total of 105,050 (with another 12,500 non-battle casualties) for the first ninety days. Marshall then contacted MacArthur again, seeking clarification as to whether these were estimates purely for medical and logistical planning or whether these were the were the expected battle result, prefacing his question with a reminder as to the President's concern over the possible level of casualties that might be sustained in Olympic. Some have interpreted this as an indirect hint to MacArthur that the previous figures were a little on the high side. MacArthur's answer arrived just in time for Marshall to use it in the 18 June meeting with Truman. In it, MacArthur downplayed the earlier figures saying that they were being used purely for planning considerations and maintained that the actual casualty figures in the campaign were likely to be substantially lower.

Nevertheless, the figures as presented by his staff were actually quite close to those from the JWPC and were consistent with an estimate as to the casualties after thirty days given by Nimitz's staff (JCS 1388/1, 20 June 1945, 'Memorandum by the Commander in Chief, US Fleet and the Chief of Naval Operations'):

The meeting with President Truman was attended by General Marshall, Admiral Leahy, Secretary of War Simpson, Assistant Secretary of War McCloy, Secretary of the Navy Forrestal, Admiral King, Lt Gen Eaker representing General Arnold and the recorded, Brig Gen McFarland. Marshall presented the report that Hull had prepared, including the casualty figures from various operations in the Pacific. The report ended with a summery that the casualty rate would be unlikely to exceed that on Luzon. Admiral Leahy challenged the Luzon comparison and indicated that he believed that the casualty rate for an attack on Kyushu would be closer to that experienced on Okinawa, noting that the casualty rate was thirty-five percent (it would be updated to thirty-nine percent in the 11 July report) and asked what the resulting casualties would be from the force to be committed against southern Kyushu. Marshall obliquely stated that the force was planned to number 766,700, although two specific documents give a ground combat force figure of about 350,000 ('Commander in Chief, Pacific Fleet, Operation Plan No. 10-45, Central Pacific Area: OLYMPIC, 8 August 1945', Naval Historical Centre and Sixth Army Field Order #74, Troop List, 28 July 1945, Records of the Strategic Plans Division, Box 187, Naval Historical Centre) the remainder presumably being made of of Army Air Force and Naval personnel. There was also some discussion as to the size of the Japanese forces that would be encountered on the island and the previous estimate of around 350,000 troops (in six combat and two depot divisions) was used. Much of the discussion surrounded the possibility of the Japanese reinforcing the garrison on Kyushu and Marshal repeatedly emphasised the impact US air and naval power was already having on the Japanese merchant and naval fleets and that the capacity to transport reinforcements would be almost negligible by 1 November. Truman's questions and comments reflect his concern over US casualties, especially as an invasion of the homeland could incite a racial desperation in the Japanese who might fight to the finish. Stimson indicated this was a distinct possibility. At the end of the meeting, Truman agreed that the plan presented was the best choice under the circumstances and then gave the go-ahead to continue preparations for the invasion of Kyushu - the decision on the follow-on Honshu operation would be delayed until a later date, the reason given being that the President and his advisors wanted to see the impact of the Kyushu invasion and Soviet entry into the war would be on the Japanese, although in the background must have lain the up-and-coming test of the atomic bomb.

As Truman was on the way to Potsdam, American Intelligence confirmed the presence of the fourth combat division on Kyushu with two more divisions forming soon after. Intercepted communications during this period showed that the US predictions were broadly on track and that the Japanese were preparing to use a large number of suicide weapons in kamikaze attacks. Continued radio traffic indicated that the Japanese were preparing additional bases for manned suicide torpedoes (kaiten) and deploying extra suicide aircraft (some 940 in July). By this time, the majority of new equipment and supplies had gone to Kyushu, hampering preparations on Honshu and Hokkaido. Despite having 100 percent of its ammunition quota, ninety-four percent of its fuel quota and 164 percent of its ration quota, it only had fifty percent of its mortar and antitank gun quota. Even the Tokyo Defence Zone was well-below its quota for supply, leading to Japanese strategists worrying that effective resistance would be impossible after the Kyushu Campaign. Many of the Japanese assumptions were not unanimous however, and debate centred around a situation report issued in July, the main issue being whether the Americans would attack as soon after the Okinawa Campaign as possible, after an extended period of blockade or after more operations on the periphery. Some were convinced the USA would attack China, while others thought they would land in southern Korea or on Saishu Island in the Korean Strait, both of which would sever Japan's supply lines the Asian mainland.

As July progressed, the Japanese force build-up detected by US Intelligence was still within the original projection but had been completed well before the anticipated invasion date of 1 November 1945 and even slightly exceeded it by 25,000 troops (375,000). Analysts gradually learnt that the fifth and sixth divisions had been on Kyushu since early May, due to US Intelligence learning a special term that was used in Japanese communications to refer to a 'division' (SRH 195, Bulletin No. 73, 28 July 1945). This therefore had meant that by the time of the 18 June meeting, Japanese forces had already reached the level predicted for the invasion, still some four months away. One can only speculate as to how this information might have impacted that meeting had it been known but in any case, the projections that had stood the test of time were about to be completely shattered by discoveries during the Potsdam Conference. On 21 July, the Military Intelligence Service's daily summery reported that three new Japanese divisions had been discovered on Kyushu with a fourth being discovered shortly after (in order of discovery, 146th, 154th, 156th and 145th). This brought the total to ten combat and two depot divisions with the possibility of another division being moved there from Honshu. This coincided with the discovery of an army-level headquarters (40th), formerly on Formosa that had been moved to southern Kyushu. A review of messages indicated that this headquarters had probably been on the island since June, confirming analyst's efforts to find such a formation due to the increasing number of divisions on the island.

By early August, Truman had arrived back from the Potsdam Conference and US Intelligence had confirmed the arrival of the eleventh division and there were indications that a further two were on their way. The intelligence data also indicated that there were a large number of specialised non-divisional units on the island, such as mixed brigades, tank regiments and artillery brigades and an increase in the number of naval ground troops defending important installations. The MIS report of 2 August estimated that enemy strength on the island had reached 534,000, although this did not include the recently confirmed eleventh division (as it had yet to deploy properly) or the two other divisions that were en-route. These three units represented around 40,000 troops. These numbers clearly demonstrated that the previously decided cut-off for the number of Japanese troops that would be deployed on Kyushu had failed to materialise and a sense of panic spread, as demonstrated by a paper circulated by the chief of MacArthur's intelligence staff, Maj Gen Charles A Willoughby on 29 July 1945 (General Headquarters, US Armed Forces Pacific, Military Intelligence Summery, General Staff 'Amendment No. 1 to G-2 Estimate of the Enemy Situation with Respect to Kyushu (dated 25 April 1945), 29 July 1945) that warned that with the continued Japanese build-up, the USA would gradually loose the favourable attack ratio that would be needed to ensure the best chance of success in an assault against the island - and this was before they had received evidence for two more divisions on the island with another two en route. The information also made it clear the build-up was primarily taking place in southern Kyushu in the area where the US planned to undertake its landings. Seven of the nine identified combat divisions and many of the independent regiments and brigades were in the south, which meant that the forces there was already double the original estimates and exceeded the estimate for the whole of Kyushu by the invasion date. Approximately 320,000 troops (about sixty percent of the total) were deployed in the south. The intelligence staff of the Sixth Army, the main ground combat formation that would be involved in the execution of Olympic, conducted an assessment (Sixth Army G-2 Estimate of the Enemy Situation; OLYMPIC Operation, RG 165, Records of the War Department, General and Special Staffs) of the Japanese build-up. They concluded that the invasion of Okinawa had persuaded the Japanese that an invasion of Kyushu would follow soon after the island was secured and so had spared no effort in order to build up the island's defences. It indicated that the increased aerial offensive would hamper Japanese efforts to reinforce their forces their but would be unlikely to stop it completely. The stark message contained in the reports and assessments by various US Intelligence staffs were flagged up in memos circulated at more senior planning levels. A summery of the Willoughby report landed on the desk of General Lincoln, the Army's senior representative on the JPS and Chief of the Strategy and Policy Group of the War Department Operations Division. The sharp increase in numbers relating to the estimates of Japanese troop strength on Kyushu were incorporated into a 4 August JWPC memo (JWPC 397, 'Alternatives to OLYMPIC', 4 August 1945) to the JPS, recommending that US Field Commanders ought to review their estimates of the situation and prepare plans for operations against alternative targets in the light of the build-up and concentration of Japanese forces on Kyushu. While this was being circulated, intercepted communications confirmed another new division had been located and on the status of the two that had previously been thought to be in transit. This brought the total number of divisions on Kyushu to fourteen, twice the original estimate. Nine of the fourteen were in or being deployed to the south, three times that originally projected raising the manpower estimate to 549,000 – a figure that would increase again to 600,000.

Allied Intelligence and the View from the Top

The intelligence picture regarding the Japanese build-up on Kyushu did not come together until after the Potsdam Conference had started and even now it is unclear as to how much information reached the US officials taking part. It is probable that the basic information would have been passed on but whether it reached the President and if so, how much and in what format, can only be inferred. US Intelligence regularly passed on information as to the appearance of new Japanese formations in signals intelligence summaries to senior officials and special channels had been set up to pass the information on to Potsdam, where a replica situation room had been set up. Marshall gave a briefing to the 'Tripartite' military chiefs on 24 July and gave the Japanese troop strength on Kyushu and the outlying Ryuku Islands (not Okinawa) as around 500,000. Official records of the discussions between Truman and his military advisors while at Potsdam make little mention of discussions regarding the expanding Japanese troop numbers on Kyushu, neither do the many diaries and memoirs produced after the war. Admiral Leahy stated that military matters occupied only a small amount of the discussions at Potsdam. It has been suggested that the absence of references to the intelligence stems from the cloud of secrecy surrounding the use of 'Ultra' and the decrypting of Axis communications. The US started out referring to Japanese high-level communications cryptanalysis as Magic while Britain referred to those from German sources as 'Ultra' but as cooperation with Britain increased, the US started to refer to the high-level intercepts as Ultra but continued to refer to the daily reports of Japanese decrypts under the heading of Magic. Those who suggest this include Edward Drea (in 'MacArthur's ULTRA') and Christopher Andrew (in 'For The President's Eyes Only') but the sensitivity to the secrecy surrounding the interception and decryption of Japanese communications did not extend completely to Stimson or Forrestal who mention it in their diaries, which were published in the early 1950s and extracts from Stimson's were incorporated into the 1960 State Department History of Foreign Relations publication on the Potsdam Conference. Truman even stated in an open conference in January 1956 that he had been away of Japan's efforts to enlist Soviet help in brokering an end to the war, even before Staling had told him. This gradually circulating knowledge regarding the Japanese efforts to build their forces up on Kyushu was reflected, as some have suggested, in a letter from Truman to an Air Force Historian (James L Cate) dated 12 January 1953, where Truman noted that at Potsdam, after he had received news about the successful Trinity atomic test in the New Mexico desert, he had asked Marshall what he thought the casualties would be if they launched both phases of Operation Downfall. The letter (as published) stated that Marshall responded that it would cost a minimum of one quarter of a million casualties and might go as high as one million. Many observers have noted that these numbers are much higher than those that Marshall had presented on 18 June, probably due to the awareness of the Japanese preparations to field a much stronger defence than had been originally forecast. It is also possible that Marshall's statement was influential in the decision to drop the atomic bomb, linking the information discovered via intelligence sources to the decision. There has been a great deal of debate over the origin and validity of the statement however, especially the calculation that the United States could suffer up a million casualties if it invaded Japan. Some seem to think that this later figure was added in an ex post facto effort to rationalise the use of the bomb and it seems that this was the case as the initial draft of Truman's response to the Air Force Historian's query only describes Marshall as saying that a quarter of a million casualties would be a minimum with the million figure being added by Truman's staff so as not to conflict with an earlier published article from Stimson (in the February 1947 issue of Harpers magazine), which itself has been challenged.

A quarter of a million casualties was roughly the figure mentioned in the JWPC in its paper prepared for the 18 June meeting, itself consistent with studies done by both Nimitz's and MacArthur's intelligence staffs. Even disregarding the upper figure of one million casualties, the figure of a quarter of a million casualties in any ground campaign in Japan may well have been a frightening prospect to many at the time. As a comparison, the combined casualty figure for the campaign through the Philippines, Iwo Jima and Okinawa was 133,000, for the Normandy campaign 63,360 and for the Battle of the Bulge 59,000. It is not unreasonable to postulate that, assuming Marshall was in possession of the latest intelligence data of what the Japanese were doing on Kyushu that he could have made such a guess but there is little evidence to say that is what he did. There remains therefore the question as to whether, and if so to what degree, the intelligence data affected the decision to use the atomic bomb. On the evening of Truman's second evening at the Potsdam Conference (16 June), he received the communiqué that the atomic bomb test at Alamogordo had been successful and received the detailed test results from General Leslie Groves on 21 June. On 24 June, General Carl Spaatz flew to Guam as head of the Strategic Air Forces in the Pacific and with written instructions for the new command to deliver the first atomic bomb (or 'special bomb' as it was known) as soon after 3 August the weather permitted. The instructions were issued with the approval of Stimson and Marshall. Spaatz received the directive while he was preparing to take command of the air forces in the Pacific. The directive was given to him by General Thomas Handy, Acting Army Chief of Staff while Marshall was in Potsdam who gave Spaatz orders to pass a copy each to both MacArthur and Nimitz. The cable that started the process was sent to Washington from Potsdam on 22 July 1945. On 31 July, Truman was given a cable from Stimson asking for approval regarding a public announcement planned to be released straight after the dropping of the first atomic bomb on Japan and apologising for his haste, which was due to the progress on the Manhattan Project being so rapid. Truman scribbled his reply (which was in the affirmative) and the cable was sent to Washington. Thus Truman gave the go-ahead two days before the MIS issued its report stating that eleven divisions were on Kyushu that was used in the JWPC's recommendation for studying alternatives to the Olympic plan and more crucially before the President had reached Potsdam.

On 1 June, the 'Interim Committee' (a group established by Truman and chaired by Stimson made up of political advisors, academics, scientists and industrialists) had recommended to the President that the new weapon be used as soon as possible on a military-industrial target in Japan, without a prior warning. This continued to be the governing principle while public statements that were to be used after the bomb were drafted and re-drafted even while debate continued over whether to provide a warning and perhaps a demonstration. Some scientists outside the committee dissented as to the use of the bomb in the first place but Truman agreed with the course of action recommended by the committee and had every intention of implementing it. Once this decision had been taken, discussion centred around how quickly the weapon could be used and whether it could be used before the formal entry of the USSR into the Pacific War, what the first target might be, what the wording would be in the 'Potsdam Declaration' that would warn the Japanese of vaguely worded 'consequences' should they fail to surrender unconditionally and what the wording would be of the President's statement immediately following the drop. The news of the test results also prompted discussion as to whether the USA still needed the USSR to come into the war at all and whether to inform Stalin of the bomb but the news of the Japanese build-up on Kyushu doesn't seem to have been specifically mention. There is also a lack of evidence concerning any sort of any formal decision-making regarding the use of the bomb, although both the 22 and 24 July have been put forward as the dates of key decisions, the first being when Truman, Churchill and their military advisors met, the second when Truman, after having received the Groves report, had a private meeting with Churchill along with Marshall and Leahy. While the record suggests there might have been a series of additional ad hoc meetings with a number of individuals, there doesn't seem to be any indications that there was a large meeting as has sometimes been claimed. Nevertheless, even if such a meeting never occurred, there is plenty of evidence to confirm that US officials were worried about the possible cost of an invasion of the Home Islands, especially in the light of the Japanese build-up on Kyushu. This however should not be viewed as the only factor prompting the decision to use the atomic bomb, which was viewed as a way of firstly, ending the war quickly to avoid the need for an invasion which would involve a level of casualties that would be intimidating by any standard used, but secondly, a way of attaining influence over the USSR in the post-war world and thirdly, a way of not having to confront the debate surrounding the issue of unconditional surrender and making possible concessions to the Japanese.

The decision to drop the atomic bomb set in chain the events that led to the unconditional surrender of Imperial Japan to the Allies, the ceremony taking place on 2 September 1945 on board the USS Missouri in Tokyo Bay. But what if the atomic bomb had not been ready? As mentioned earlier, there had been a recommendation from the JWPC in a memorandum dated 4 August 1945. In it was a draft cable that the committee had recommended be sent to both Nimitz and MacArthur, referring to highly classified reports that indicated that the Japanese were strengthening the defences on Kyushu beyond what had previously been expected in the run up to the Olympic target date. The two commanders, while not required to alter their plans at that precise point in time, were encouraged to make alternate plans in the light of the additional planning that was being undertaken at alternative targets such as north or northeast Honshu or directly against the Tokyo Plain. Such a question came before Marshall on 6 August (the day the first atomic bomb was dropped) who was notified that the next meeting of the JCS would probably address the issue of the Japanese build-up on Kyushu and examine possible alternatives to Olympic. It is reasonable to assume therefore that had the bomb not been dropped, this meeting would have been held. If so, it is entirely probable that the earlier arguments put forward by Admiral King and General Arnold to continue the strangulation of Japan by air bombardment and sea blockade would have surfaced once again, this time supported by the signals intelligence from Kyushu. While there is no indication that any previously produced casualty estimates had been revised due to the intelligence about the Japanese build-up on Kyushu, any future meeting of the JCS would not have been able to duck the issue that such a build-up would have implications for the potential casualties that would be incurred. The original estimates had been produced by the JWPC and operational staffs when the forecast had estimated that the enemy forces that were to defend the island were only half the size they had reached when the war ended, and even these estimates had been withheld from the 18 June meeting with President Truman.

The most recent experience that could bear any relation to what might be waiting to greet the US was the recently (mostly) concluded battle for Okinawa that had seen some 75,000 regular Japanese Army troops, along with about 25,000 local militia, inflict around 49,000 casualties on US forces. The invasion of Kyushu would have meant an operation aimed at an island several times larger than Okinawa with even the southern half, the projected area for the Olympic operation, being twice the size. Kyushu was initially expected to be defended by a Japanese force about three-and-a-half times the size of the one on Okinawa but by early August was garrisoned by one over six times the size while the initial casualty estimates forecast American casualties at just over 100,000, only twice those suffered on Okinawa. Intelligence had indicated that the Japanese were preparing to use similar tactics as those used on Okinawa including suicide attacks and guerrilla warfare with the number of US troops that were to be involved in Operation Olympic as about 350,000 - initially anyway - about three times that involved in Operation Iceberg. These figures would have given some senior members of the decision-making process, Admiral Leahy among them, a strong case that the estimated casualty rates would need to be revised upwards in order to remain credible. Leahy had earlier supported the bomb-and-blockade strategy and was recorded in the minutes of a White House meeting that unconditional surrender was not worth the price in American casualties. On the other side of the argument would have been (in all likelihood) General Douglas MacArthur who favoured proceeding with the invasion as planned. When told that the main issue that would be discussed at the forthcoming JCS meeting would be the Japanese build-up on Kyushu, Marshall cabled MacArthur to solicit his views, emphasising that the numbers, as reported by intelligence, would pose a grave risk to US amphibious assault forces. Noting that the build-up on Kyushu had been augmented at the expense of the preparations in other areas, Marshall asked MacArthur about possible alternative objectives at less defended sites, such as the three sites that had been mentioned in the JWPC's 4 August memo. MacArthur was typically dismissive of the build-up in that he 'did not credit' the reported strength of forces in southern Kyushu, was adamant that continued airstrikes would impair the Japanese ability to deploy reinforcements (despite evidence to the contrary) and dismissed the alternatives mentioned by Marshall as impractical without airbases nearer the mainland or requiring a substantial time delay. He also argued that the basic plan for Olympic was sound and was confident it would succeed - in many instances throughout Southwest Pacific campaign he had received intelligence that that the Japanese were building up their forces but this intelligence had always been found to be faulty. While he did not argue that the build-up, if true, was not a threat to the invasion, he attempted instead to cast doubt on the accuracy of the intelligence data. In this he had already shown a tendency to ignore intelligence estimates from his own G-2 where it interfered with his aims, and while such reports had quite often been wrong, the inaccuracies had generally quite small.

A middle ground between the two extremes would have been the approach favoured by the JWPC with some support from Marshall - an invasion of Japan but at a less well-defended site. The views of General Marshall and many on the JWPC on the desirability of unconditional surrender would mean that it would have been difficult to back away from advocating an invasion of the Japanese Home Islands. The terms of the unconditional surrender involved much more than just the status of the Emperor, although this in the end proved to be the easiest to resolve. Many governments, including the Australian and Chinese, were opposed to the retention of the Imperial dynasty but many key officials in the US and UK were willing to allow it to continue as it would facilitate the administration of the country in the post-war world. The other Allied objectives included the unrestricted occupation of Japanese territory, total authority in the running of the country, the dismantlement of Japans armed forces and military industrial complex, the restructuring of Japanese society and an Allied-run war crimes trial - doing to Japan what was at the time being done to Germany. Abandoning unconditional surrender would mean that Japan would not suffer the same consequences as Germany and Truman knew what the political consequences of that would be. He would leave the possibility open for a modification of the surrender terms but the initiative would have to come from Congress. The only problem was that it would be difficult to achieve the surrender of an entire nation steeped in a warrior tradition and having a history as a great power without having captured any of that nation's territory. It had not been achieved in the case of Germany without an invasion. This was the argument between those who advocated an invasion and a blockade. The potential casualty costs of an invasion would have added strength to the bomb-and-blockade argument but the downside of that was time - how much of Japan would have to be destroyed, over how much time in order to kill how many thousands of Japanese either by bombing or starvation, before unconditional surrender would be accepted. Added to this, there was also the question of the Soviet entry into the war. Such an event could trigger Japanese surrender but on the other hand, if Japanese surrender did not occur until after the Soviets had been involved in the Pacific War for sometime, it would be uncertain that the USA and its allies could acquire the control over Japan that they required without having to include the Soviets, especially if there had been no US ground invasion of the Home Islands. Such an inclusion it was feared, could result in additional costs, as had been seen in the consolidation of Soviet hegemony in Eastern Europe that was beginning to be seen.

Such considerations supported the idea that the USA should still seek to invade and occupy territory on the Japanese mainland but that alternative sites to Kyushu should be found. As well as the options outlines in the JWPC memo of 4 August, there would have been the option of keeping Kyushu as the main target but postponing the ground invasion until the airbases that were being built on Okinawa had been finished and the additional airpower be used in an extended campaign to destroy Japanese defences, troop concentrations, bases and infrastructure. Such an intensified air campaign had been slated to begin in mid-September but had been brought forward thirty days due to Willoughby's recommendation vis-à-vis the build-up on Kyushu. Any changes in the choice of invasion site would have to be tempered by weather and timing considerations. Olympic had already been moved forward a month from a 1 December 1945 start date due to concerns (expressed by both MacArthur and Nimitz) over the potential problems which might be caused by bad weather if the invasion started at that time of year. Such a change in plans could have caused delays which might have set the invasion schedule back to early 1946 and the date for Coronet back even further, the timing of which was crucial so as to avoid the rain storms and maximise the possible use of armoured units on the Tokyo Plain. If this was the case, there seems little difference between that and the bomb-and-blockade strategy - each would require the invasion be put on hold (temporarily anyway) and intensifying the air and sea attack on Japan. If that did not produce a Japanese surrender in about six months then the invasion issue would have to be reconsidered. The military alternative was to go ahead with the invasion as planned (perhaps augmenting the attack with additional ground and air forces) and risk high casualties, while the political alternative was to relax the terms of surrender. This sort of dilemma was exactly what the Japanese had hoped to stimulate in the Allies. They wanted to buy enough time to encourage war-weariness in the Allied civilian populations in combination with increasing concern about high casualties that might force a softening of the unconditional surrender terms. By this stage in the war, even for the Japanese it was not a case of whether they would surrender but on what terms as the best leverage the Japanese had, was the ability to influence the perceptions of cost (both military and political) for the Allied decision-makers. The downside for the Japanese was that in so effecting Allied perceptions and dragging out the decision-making cycle and therefore the war, the cost to them would be that much higher in terms of loss of life and destruction of property. In addition, the USSR would become more involved and for longer and the chances of achieving an acceptable post-war settlement once the surrender did come would be more difficult.   Any guess as to how the debate over whether to invade or not between the Allied leaders is a matter of personal choice, but whatever the result would have been, had the bomb not been available when it was, the JCS meeting scheduled for 6 August 1945 would have been pivotal and with it the contribution of signals intelligence, to the course of the Pacific War.

World War Two: U.S. Military Plans for the Invasion of Japan, Thomas Fensch (Editor) This is a very useful collection of official American documents relating to Operation Downfall, the plan for the invasion of Japan. The invasion never needed to be carried out, but the plans had reached a very advanced stage by the time the two Atomic bombs ended the war. [see more]

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