The First Consul, who had left Milan on the morning of the 9th of June, arrived at Montebello just at the termination of the battle. Expecting that Melas would at once advance with all the troops that he had collected at Alessandria, Bonaparte began on the 10th of June to rearrange his troops, and to make preparations for battle. Being deficient in both cavalry and artillery, while Melas was well supplied with both, Bonaparte decided to fall back to a position near Casteggio, in front of the Stradella Pass, where his flanks would be protected by the Po on one side, and by the spurs of the Apennines on the other. With the corps of Lannes and Victor he made a retrograde movement to this point. Here he collected all his forces south of the Po, now numbering twenty-nine thousand men. In this strong position he remained for several days, expecting hourly that the Austrians would push forward from Alessandria and attack him. But they failed to appear.
On the 11th of June General Desaix, who had served under Bonaparte in Egypt, arrived at the French headquarters. He was a distinguished general, and a warm friend of the First Consul. At once Bonaparte gave him the command of a corps, consisting of two divisions.
On the following day Bonaparte, surprised at the non-appearance of the Austrians, began to fear that they were trying to escape. He thought that Melas might attempt to evade him, either by marching directly on Genoa, or by crossing the Po at Valenza, and thence marching on Pavia and Milan. Finally, he could bear the suspense no longer. He decided to advance and seek Melas. Accordingly, on the afternoon of the 12th of June, having left a force to occupy the intrenched camp at the Stradella Pass, he advanced towards Alessandria. At Tortona he left a force to blockade the fortress. On the 13th of June he crossed the Scrivia and debouched into the plain of Marengo, which lies between the Scrivia and Bormida rivers. Thus far he had met with no Austrians. His anxiety increased. He had but few cavalry, and, consequently, was unable to make a thorough reconnoissance of the surrounding country. During the afternoon of that day, he directed Victor on Marengo. Here the French found only a small detachment, which was quickly driven across the Bormida. A party sent forward to reconnoitre the crossings of the Bormida, reported that no Austrians were to be found there in force.