Had, therefore, the Army of Reserve been defeated while marching from Ivrea to Milan, it would have lost its line of communication by way of the Great St. Bernard. In that case it would undoubtedly have been captured or destroyed; for since the Italian entrances of the St. Gothard and Simplon passes were then held by ten thousand Austrians under Vukassovich, Bonaparte could not have retreated into Switzerland. That Bonaparte appreciated the critical features of the situation is seen in the skill with which he planned and executed the march. By ordering Lannes to make preparations to cross the Po at Chivasso, Bonaparte gave Melas the impression that the French intended to cross the Po and attack the Austrians near Turin. Thus Melas was deceived. Meanwhile Bonaparte, with the greater part of the Army of Reserve, marched rapidly on Milan. During the march Lannes descended the Po towards Pavia, thus covering as with a screen the movements of Bonaparte. So skilfully were these manoeuvres made that Melas did not even attempt to cross the river, in fact, did not even learn of the march of Bonaparte until the 29th of May, two days after the movement had begun. On the 31st Bonaparte arrived on the Ticino; and on the 2d of June, having driven back Vukassovich's corps, he entered Milan. Here the critical part of his march ended, for he was then sure of being joined by Moncey's corps, and had, in case of need, a safe line of retreat into Switzerland by the St. Gothard and Simplon passes. Another circumstance that aided Bonaparte in this march was the presence of Thurreau's division of four thousand men at Susa. Melas, being ignorant of the strength of this division, hesitated to push forward and attack Lannes, so long as these troops remained undefeated on his flank and rear. On this point General Hamley makes the following comments:-- "Thurreau's force, being entirely separated from the main army throughout the operations, was useful only as leading the enemy to a false conclusion. But its value in that respect was incalculable. There were sufficient Austrian troops round Turin to check Thurreau and crush Lannes, thus laying bare the rear of the French army. But the road of the Mont Cenis was both more practicable and more direct than that of the St. Bernard; moreover, Thurreau had artillery, and Lannes, at first, had not, for his guns had been delayed by the difficulties of passing the Austrian fort of Bard. It was but a natural error, therefore, for Melas to believe that Thurreau was backed by the whole French army."