The siege of Alessandria (25-29 August 1499) was the main military action during the French invasion of Milan in 1499, and the fall of the city helped force Ludovico Sforza to flee from Milan (Second Italian War/ Italian War of Louis XII).
Louis XII came to the throne in 1498, and spent the first year of his reign preparing for an invasion of Milan. Ludovico Sforza, duke of Milan, was well aware of what was coming, but his preparations were rather less effective than the French. His main achievement was to improve the defences of Alessandria, which was judged to be a very strong fortress. Command of the garrison was given to Galeazzo San Severino, one of his closest allies.
By mid-August 1499 the French army of around 26,000 men, commanded by Gian Giacomo Trivulzio, was at Asti, and the invasion began. The French occupied Annone, Valenza and Tortona, and then on 25 August arrived outside Alessandria.
Galeazzo found himself in a very difficult position. His troops hadn’t been paid, his own brothers were unreliable and he was almost isolated in the face of a much superior army. On 29 August Galeazzo fled from the city, and it immediately fell to the French.
The fall of Alessandria fatally undermined Ludovico's position in Milan. On 2 September he left the city and fled into the Tyrol, where he hoped to final allies. The French quickly occupied Milan, and Louis made his entry into the city on 6 October 1499. Ludovico was able to raise an army, and made an attempt to regain his duchy in 1500, but his army dissolved at Novara and he fell into French captivity.