The combat of Marengo (13 June 1800) was a minor French victory on the evening before the battle of Marengo that badly disrupted the Austrian plans for the following day by giving the French command of a crucial bridge in the village of Marengo. After a month that had seen the French and Austrians manoeuvre around large parts of northern Italy (Marengo Campaign), the two armies approached each other to the east of Alessandria. The Austrian army was camped between Alessandria, and the Bormida River, just to the east. They had a fortified bridgehead over the Bormida just to the east of the city, while their advance-guard, under O'Reilly, occupied the village of Marengo, guarding a key bridge over the Fontanone. The Austrian commander, Michael Friedrich Freiherr von Melas, hoped to use a double-agent to convince the French that he was about to attempt a breakout to the north across the Po. Napoleon would move to Sale, close to the Po to counter this move. Melas would then cross the Bormida close to Marengo and attack the left wing of Napoleon's army. This would open up the road east to Mantua and allow Melas to escape from Napoleon's trap and reunite his scattered armies.
Napoleon was not entirely convinced by the Austrian plan, but he did dismiss any idea that Melas might be planned an attack to the east. On 13 June he split his army, sending Desaix south to watch for any attempt to escape to Genoa, and other forces north towards the Po to watch for any breakout to the north. Most of his army did march through Sale, but then turned south, and took up a position on the road that ran west from Tortona towards Marengo. The French discovered O'Reilly's troops at Marengo, but believed them to be the Austrian rear-guard not the advance-guard.
Napoleon decided to attack O'Reilly's men that evening, in an attempt to pin them down. Victor's corps was sent west along the road from Tortona. In mid-afternoon the French reached a fort in the road. Gardanne's and Chambarlhac's divisions were sent along the Old Road, towards Marengo, while Dampièrre, with part of the 44th demi-brigade and the 12th Chasseurs à Cheval, was sent down the New Road, which branched off to the left.
At about six in the evening the French attacked O'Reilly's lines at Marengo. Gardanne made a bayonet attack on the village, which encountered some stiff resistance. This ended when O'Reilly saw Dampièrre's column approaching from the south. Their line of advance had taken them across the Fontanone to the south of the Austrian positions, and they were now advancing towards his exposed right flank. O'Reilly realised that he would have to retreat before he was cut off.
Gardanne followed the Austrians as they pulled back, and took up a new position at Pedrabona Farm, close to the Bormida. The French then advanced towards the river, a move that might have brought them up to the Austrian bridgehead. To prevent this IR 51 Splenyi and the 10. Lobkowitz Dragoons were sent across the bridge to attack Gardanne. Supported by artillery from three batteries on the opposite bank of the river and from the bridgehead they were able to force the French back to Pedrabona.
Although the combat of Marengo was only a minor clash, it badly disrupted the Austrian plans. Even if Napoleon's main forces had still been around Sale, the Austrians would now have to fight their way across the Fontanone before they could begin their attack on the French left. Poor French reconnaissance meant that they missed an even bigger chance to spoil Melas's day. French scouts reported that the Austrians didn't have any bridges over the Bormida, and that there was only a garrison in Alessandria. Melas's main army, camped on the west bank of the river by the bridgehead, was missed completely. The Austrians would be free to launch their surprise attack on the French on the next morning.