Napoleon Bonaparte invaded Italy across the Great St Bernard Pass in May 1800 and seized Milan on 2 June. Reacting to the surprise French offensive, Melas ordered O'Reilly to take 3,000 troops and an artillery convoy to defend Piacenza. The Austrian army commander hoped to use Piacenza as a base to launch a counterattack against Milan. In the event, Piacenza fell to the French before O'Reilly reached it. On 6 June, after an 11-hour skirmish, he successfully managed to save the artillery convoy and withdraw to the west, while fending off a French column that tried to cut him off.