The battle of Cassano (27 April 1799) was an Austro-Russian victory outside Milan that saw them force their way across the River Adda, making the fall of the city inevitable.
At the start of the War of the Second Coalition the French and Austrians had faced each other across the Adige River, but the French suffered two defeats while attempting to capture Verona (battle of Verona, 26 March 1799 and battle of Magnano, 5 April 1799), and were forced to retreat towards Milan. General Schérer was replaced by Moreau, whose first task was to defend the line of the Adda River.
In mid April General Kray's Austrians were joined by Field Marshal Suvorov's Russians, and the Allied army advanced west towards the Adda and Milan. The French were spread out along a very long front, from Lecco to Pizzighettone close to the Po, and the defeats on the Adige meant that Moreau only had 27,000 men, while the Allies had around 50,000 men free to move west.
Fighting began on 26 April at Lecco, between the south eastern tip of Lake Como (also known as Lake Lecco at this point) and Lake Garlate. A Russian force under Peter Bagration managed to cross the river but was then held up by the French garrison under General Soyez. After several hours Bagration was ordered to move south to Brivio, at the southern of Lake Garlate, where an Austrian force under General Vukassovich had already crossed the river.
The main Allied attack came on 27 April, at Trezzo and Cassano d'Adda. Three Allied divisions were concentrated at San Gervasio, opposite Trezzo (One Russian division under Franz Fürst Rosenberg and two Austrian divisions under Generals Zopf and Ott). On the night of 26-27 April Allied engineers built a pontoon bridge over the river, and on the next day the three divisions crossed the river and began to push back Grenier's division. Later in the day General Victor arrived with reinforcements, but he was unable to stop the Allied advance, and by the end of the afternoon the French were forced to retreat back towards Milan.
Further south an Austrian force under Michael Freiherr von Melas was held up by the 106th Demibrigade, which was defending Cassano on the western bank of the river. Only when Field Marshal Suvorov arrived to watch the fighting were the Austrians able to push the French out of their entrenched positions at Cassano.
The Allies were now across the river on a fifteen mile long front, from Brivio in the north to Cassano in the south. Most of the French army was retreating back towards Milan, but Sérurier's division was left behind at Paderno (between Brivio and Trezzo). On the day after the battle Sérurier was surrounded and was forced to surrender to General Vukassovich.
The Austrians and Russians suffered 4,886 casualties at Cassano (761 dead, 2,913 wounded and 1,212 prisoners). The French lost at least 5,000 men, while some estimates go as high as 6,900. Although most of the French army thus escaped intact, Moreau was unable to hold Milan, and on 29 April the Allies captured the city.