The battle of Castiglione (5 August 1796) was a French victory that effectively ended the first Austrian attempt to lift the siege of Mantua, and was an early example of a battle in which Napoleon brought several different columns together on the same battlefield. At Castiglione the Austrians had 25,000 men, the French around 28,000 men, even though Würmser had begun the campaign with well over 40,000 men.
The Austrian army, commanded by General Würmser, began its advance in three columns. Würmser himself commandeered the main central column, which advanced down the Adige valley. The second column, on the left, advanced from the north east, and soon joined Würmser around Verona. Finally the third column, under General Quosdanovich, advanced down the Chiese valley, on the western shores of Lake Garda.
This plan meant that Napoleon was able to deal with the two main Austrian forces before they could unite against him, but only because Würmser made a disastrous mistake. At first the Austrian campaign progressed as planned. On 28 July Würmser and Quosdanovich emerged from the mountains and forced the French advanced guards back. On 30 July Quosdanovich captured Brescia, to the west of the lake, blocking Napoleon's lines of communication west to Milan. Napoleon decided to concentrate all of his forces south of Lake Garda, and on 31 July won a minor victory over part of Quosdanovich's force at Lonato (first battle of Lonato, 31 July 1796).
At this point the two Austrian armies were quite close together, and if Würmser had turned west they would probably have been able to unite, giving them more men than Napoleon. Instead Würmser continued on south east to Mantua, reaching the city on 2 August. Although this did allow him to throw fresh supplies into the city, it also allowed Napoleon to defeat an attempt by Quosdanovich to fight his way through to Würmser (second battle of Lonato, 3 August 1796). Even after his diversion to Mantua, Würmser's advance guard was close enough to force Napoleon to deploy Augereau's division against him, in what is sometimes called the first battle of Castiglione (3 August).
The main battle took place of 5 August. Würmser rushed north from Mantua in an attempt to link up with Quosdanovich, while Napoleon gathered Masséna's and Augereau's divisions at Castiglione. By the morning of 5 August the Austrians had taken up a defensive position on a ridge of higher ground that runs east from Castiglione to Solferino. The main Austrian line was on the hills above Solferino, but an outlying force held a smaller rise at Monte Medolano, south west of Solferino.
Napoleon planned to use Masséna and Augereau to launch feints against the centre of the Austrian line, with the hope that Würmser would either extend his lines or weaken his flanks. He would then attack both Austrian flanks with forces that began the day some way off the battlefield. General Despinois, from Brescia (11 miles to the west) was to hit the Austrian right wing, while General Sérurier's division (temporarily under the command of General Pascal-Antione Fiorella), coming from Mantua, was to hit the Austrian rear-left.
The battle did not unfold entirely as Napoleon had planned. Würmser didn't fall into Napoleon's trap, and didn't need to deploy his reserves to deal with Masséna and Augereau. When the fighting starting the French had just under 20,000 men on the battlefield, the Austrians 25,000. Napoleon would only have a numerical advantage once his flank attacks began. Napoleon's next move was to send his reserves (commanded by General Charles Kilmaine with 900 cavalry under General Marc-Antoine de la Bonninière and 800 infantry under General Claude Verdier supported by 12 guns under Auguste Marmont) to attack Monte Medolano. This attack was successful, but the Austrian line was still intact.
Soon after this Fiorella's men arrived from the south east. Würmser was able to deploy three battalions from his second line to deal with this new threat, and the Austrian line still held. Napoleon launched another attack in the centre, which was supported by Despinois's division, which had now arrived from Brescia. This attack finally forced the Austrians out of their positions around the La Rocca tower above Solferino, and threatened to split the Austrian line. There was now a real chance that Würmser's line of retreat east to the Mincio would be cut, but luckily for the Austrians reinforcements arrived under General Weidenfeld, and Würmser was able to mount an organised retreat across the river. The Austrians lost around 3,000 men and 20 guns at Castiglione (some estimates are higher), while the French suffered around 1,500 casualties.
The French were unable to pursue, partly because of a lack of cavalry and partly because of exhaustion after a long day of marching and fighting. For a short time Würmser hoped to hold the line of the Mincio, but Napoleon quickly forced him to retreat back into the Tirol. The first attempt to raise the siege of Mantua had failed. Napoleon was able to re-establish the siege, but the French would not be left undisturbed for long. At the start of September Würmser would return to make a second attempt to lift the siege.