The Army of the Reserve was joined by Napoleon, and in mid-May set out to cross the Alps to attack the Austrian rear. The bulk of the army crossed by the Great St Bernard Pass still covered by snow. Artillery was manhandled over with great effort and ingenuity. On 14 May 1800, the 40,000-strong French army was stopped by 400 Austro-Piedmontese soldiers at Fort Bard in the Aosta Valley. They held the pass for two weeks, completely ruining Napoleon's surprise attack on the Po Valley.