The fighting in north-east France was indecisive during January and the first week of February. During the Battle of Brienne (29 January 1814) Napoleon surprised Blücher at his headquarters and nearly captured him. Having learnt that Napoleon was at hand Blücher fell back a few miles to the east the next morning to a strong position covering the exits from the Bar-sur-Aube defile. There he was joined by the Austrian advance guard and together they decided to accept battle—indeed they had no alternative, as the roads in rear were so choked with traffic that retreat was out of the question. At about noon on 2 February Napoleon attacked them opening the Battle of La Rothière. The weather was terrible, and the ground so heavy that the French guns, the mainstay of Napoleon's whole system of warfare, was useless and in the drifts of snow which at intervals swept across the field, the columns lost their direction and many were severely handled by the Cossacks. Although the French inflicted more damage than they received, Napoleon retired to Lesmont, and from there to Troyes, Marshal Marmont being left to observe the enemy.