Thus the Army of Reserve crossed the Alps. By the 20th of May all five corps had reached the valley of the Aosta. Owing to the careful preparations made, there had been scarcely any accidents and no serious delays during the passage. But the greatest difficulty was yet to be met. Some distance down the valley of the Aosta, upon a perpendicular rock commanding a narrow defile, Fort Bard had been constructed. Though this fort was garrisoned by only two or three hundred Austrians, it was impregnable and controlled the whole valley. After descending the Great St. Bernard Pass, Lannes had pushed on down the valley, but was stopped by the fire of the fort. At once he made an effort to capture the place, but was repulsed. He soon saw that it could not be taken by force. Though he gained the road that led past the fort, the deadly fire of the Austrians prevented him from advancing. For a time it seemed that this small but formidable fort would stop the progress of the whole army. Lannes was greatly disturbed. He reported the matter to Berthier, and Berthier sent at once a courier to inform Bonaparte of the situation. The First Consul was still at Martigny, where he had remained for the purpose of hastening forward all the artillery and the rear divisions of the army. This news was a complete surprise to Bonaparte. The effect which it produced upon him is thus described by Thiers:-- "This announcement of an obstacle, considered insurmountable at first, made a terrible impression on him; but he recovered quickly, and refused positively to admit the possibility of a retreat. Nothing in the world should reduce him to such an extremity. He thought that, if one of the loftiest mountains of the globe had failed to arrest his progress, a secondary rock could not be capable of vanquishing his courage and his genius. The fort, said he to himself, might be taken by bold courage; if it could not be taken, it still could be turned. Besides, if the infantry and cavalry could pass it, with but a few four-pounders, they could then proceed to Ivrea at the mouth of the gorge, and wait until their heavy guns could follow them. And if the heavy guns could not pass the obstacle which had arisen, and if, in order to get any, those of the enemy had to be taken, the French infantry were brave and numerous enough to assail the Austrians and take their cannon. "Moreover, he studied his maps again and again, questioned a number of Italian officers, and learning from them that many other roads led from Aosta to the neighboring valleys, he wrote letter after letter to Berthier, forbidding him to stop the progress of the army, and pointing out to him with wonderful precision what reconnoissances should be made around the fort of Bard."