In the spring of 1800, Napoleon and his troops crossed the Swiss Alps into Italy, aiming to surprise the Austrian armies that had reoccupied the peninsula when Napoleon was still in Egypt.[note 5] After a difficult crossing over the Alps, the French army entered the plains of Northern Italy virtually unopposed. While one French army approached from the north, the Austrians were busy with another stationed in Genoa, which was besieged by a substantial force. The fierce resistance of this French army, under André Masséna, gave the northern striking force precious time to carry out their operations with little interference. After spending several days looking for each other, the two armies finally collided at the Battle of Marengo on June 14. General Melas had a numerical advantage, fielding about 30,000 Austrian soldiers while Napoleon commanded 24,000 French troops. The battle began favorably for the Austrians as their initial attack surprised the French and gradually drove them back. Melas concluded that he'd won the battle and retired to his headquarters around 3 pm, leaving his subordinates in charge of pursuing the French. However, the French lines never broke during their tactical retreat; Napoleon constantly rode out among the troops urging them to stand and fight. Late in the afternoon, a full division under Desaix arrived on the field and dramatically reversed the tide of the battle. A series of artillery barrages and fortunate cavalry charges managed to decimate the Austrian army, which fled chaotically over the Bormida River back to Alessandria, leaving behind 14,000 casualties. The following day, the Austrian army agreed to abandon Northern Italy once more with the Convention of Alessandria, which granted them safe passage to friendly soil in exchange for their fortresses throughout the region.