Surprised by the Austrian advance toward Genoa in mid-April 1800, Bonaparte hastily led his army over the Alps in mid-May and reached Milan on 2 June. After cutting Melas’ line of communications by crossing the River Po and defeating Feldmarschallleutnant (FML) Peter Karl Ott von Bátorkéz at Montebello on 9 June, the French closed in on the Austrian army, which had massed in Alessandria. Deceived by a local double agent, Bonaparte dispatched large forces to the north and south, but the Austrians launched a surprise attack on 14 June against the main French army under Gen. Louis Alexandre Berthier.
Initially their two assaults across the Fontanone stream near Marengo village were repelled, and Gen. Jean Lannes reinforced the French right. Bonaparte realized the true position and issued orders at 11:00 am to recall the detachment under Général de Division (GdD) Louis Desaix, while moving his reserve forward. On the Austrian left Ott’s column had taken Castel Ceriolo, and its advance guard moved south to attack Lannes’ flank. Melas renewed the main assault and the Austrians broke the central French position. By 2:30 pm the French were withdrawing and Austrian dragoons seized the Marengo farm. Bonaparte had by then arrived with the reserve, but Berthier’s troops began to fall back on the main vine belts. Knowing Desaix was approaching, Bonaparte was anxious about a column of Ott’s soldiers marching from the north, so he deployed his Consular Guard infantry to delay it. The French then withdrew steadily eastward toward San Giuliano Vecchio as the Austrians formed a column to follow them in line with Ott’s advance in the northern sector.
Desaix’s arrival around 5:30 pm stabilized the French position, as the 9th Light Infantry Regiment delayed the Austrian advance down the main road and the rest of the army reformed north of Cascina Grossa. As the pursuing Austrian troops arrived, a mix of musketry and artillery fire concealed the surprise attack of Général de Brigade (GdB) François Étienne de Kellermann’s cavalry, which threw the Austrian pursuit into disordered flight back into Alessandria, with about 14,000 killed, wounded or captured. The French casualties were considerably fewer, but included Desaix. The whole French line chased after the Austrians to seal une victoire politique (a political victory) that secured Bonaparte’s grip on power after the coup. It would be followed by a propaganda campaign that sought to rewrite the story of the battle three times during Napoleon’s rule.
The Battle of Marengo was the victory that sealed the success of Bonaparte's Italian campaign of 1800 and is best understood in the context of that campaign. By a daring crossing of the Alps with his Army of the Reserve (officially commanded by Louis Alexandre Berthier) in mid-May 1800 almost before the passes were open, Bonaparte (who crossed on a mule) had threatened Melas' lines of communications in northern Italy. The French army then seized Milan on 2 June, followed by Pavia, Piacenza and Stradella, Lombardy, cutting the main Austrian supply route eastward along the south bank of the Po river. Bonaparte hoped that Melas' preoccupation with the Siege of Genoa, held by Gen. André Masséna, would prevent the Austrians from responding to his offensive. However, Genoa surrendered on 4 June, freeing a large number of Austrians for operations against the French.
On 9 June Gen. Jean Lannes beat Feldmarschallleutnant Peter Ott in the Battle of Montebello. This caused Bonaparte to get overconfident. He became convinced that Melas would not attack and, further, that the Austrian was about to retreat. As other French forces closed from the west and south, the Austrian commander had withdrawn most of his troops from their positions near Nice and Genoa to Alessandria on the main Turin-Mantua road.