However, Melas had not raised the siege of Genoa, and on 4 June the French Army marched out with all equipment towards France as per the terms of the negotiated evacuation of the city. Napoleon then faced the possibility that, thanks to the British command of the Mediterranean, far from falling back, the Austrians could instead take Genoa as their new base and be supplied by sea. His defensive posture would not prevent this; he had to find and attack the Austrians before they could regroup. He therefore advanced from Stradella towards Alessandria, where Melas was, apparently doing nothing. Convinced that Melas was about to retreat, Napoleon sent strong detachments to block Melas's routes northwards to the Po, and southwards to Genoa. At this point, Melas attacked, and for all the brilliance of the previous campaign, Napoleon found himself at a significant disadvantage in the consequent Battle of Marengo (14 June). Napoleon was effectively defeated in a tough battle in the morning and early afternoon; Melas, thinking he had already won, had turned over delivery of the coup de grace to a subordinate, when the prompt return of a detached French force under Desaix and a vigorous French counter-attack (in the course of which Desaix was killed) converted the battle into an important French victory.