The Third Diadoch War saw the struggle between the successors of Alexander the Great become a struggle to prevent Antigonus Monophthalmus from reuniting Alexander’s empire. At the end of the Second Diadoch War Antigonus had defeated Eumenes of Cardia in a campaign that had started in Asia Minor, passed through Syria and ended in Iran. As a result, Antigonus had found himself in charge of a vast portion of Alexander’s empire, and wasted no time in establishing his control over this area. A number of existing satraps were replaced by supporters of Antigonus, while he took 25,000 talents from the treasuries of Ecbatana, Persepolis and Susa. This combined with the money he had seized from Eumenes and a tribute from Iran made his the wealthiest of the successors, allowing him to maintain a large mercenary army.
In 315 he turned his attention towards Seleucus, the satrap of Babylon. Alarmed, Seleucus fled to Ptolemy in Egypt, and warned him of Antigonus’s ambitions. This can hardly have come as a surprise to Ptolemy, or to the remaining successors, for in the same year Ptolemy, Cassander and Lysimachus issued an ultimatum to Antigonus. In it they demanded that he return Syria to Ptolemy, allowed Seleucus to return to Babylon, give Hellespontine Phrygia to Lysimachus and possibly give Cappadocia and Lycia to Cassander. He was also ordered to split Eumenes’s treasury between them all. This would have left Antigonus with part of Asia Minor. Unsurprisingly he refused.
Antigonus first concentrated on Phoenicia, besieging Tyre, then advancing south to capture Joppa and Gaza. The siege of Tyre would last until 314 BC, but while he was at Tyre Antigonus issued a declaration that would play a major role in the events of the next fifteen years. After condemning Cassander, he declared his support for the freedom of the Greek cities. They were to be autonomous, and un-garrisoned (but not untaxed). This was partly aimed against Cassander, who held many of the cities of mainland Greece, but also covered the widespread Greek cities to be found in Asia Minor. It would play an important role in the Fourth Diadoch War.
Antigonus did make an effort to establish himself in Greece during this war. He made peace with Polyperchon, and sent troops and men to Greece, under the command of his nephew Polemaeus. He also probably helped found the League of the Nesiotes, which would become a significant force in the Aegean. Antigonus himself took command of the war in Asia Minor, leaving his son Demetrius in charge in Syria.
Ptolemy took his time to act, but when he did, in 312, he won a major victory over Demetrius at Gaza. After this victory Seleucus was able to return to Babylon, where he quickly seized power. Antigonus was faced by the potential collapse of his position in the east. He had already been involved in negotiations with Cassander and Lysimachus, and in 312 they came to an agreement that effectively reflected the situation at the start of the war. Ptolemy was now faced with the prospect of being isolated against Antigonus, and quickly joined the negotiations. The peace was formally agreed in 311 BC.
The peace treaty effectively restored the pre-war position. Cassander retained Macedonia, Lysimachus Thrace, Ptolemy Egypt and Antigonus Asia. Seleucus was not included in the peace, and would have to fight on against Antigonus until 308 (Babylonian War). One final casualty of the war was Alexander IV, Alexander the Great’s thirteen year old son. Cassander had been recognised as “general of Europe” until Alexander came of age. In order to prevent that, in 310 he had Alexander and his mother murdered. The successors were now free to claim the throne for themselves, although they would not begin to do so until 306.