10 April, leading elements of the Austrian army crossed the Inn River into Bavaria; there was no formal declaration of war. Bad roads and freezing rain slowed the Austrian advance in the first week, but the opposing Bavarian forces of Lefebvre's corps gradually retreated. Davout's III Corps withdrew westerwards towards Ingolstadt, anticipating orders to concentrate with other French forces. The Austrian attack had occurred about a week before Napoleon anticipated, disrupting French plans. Napoleon ordered that an Austrian attack before 15 April should be met by a general French concentration around Donauwörth and Augsburg in the west, but his orders arrived fragmented and out of sequence and were poorly interpreted by Berthier who was more accustomed to staff duties than field command. Berthier focussed on an ambiguous sentence that called for Davout to station his III Corps around Regensburg "whatever happens"; it is likely that Napoleon intended this to apply only if the Austrians attacked after 15 April. On 14 April Berthier ordered Davout's corps, together with those of Lefebvre and Oudinot, to march to Regensburg which Davout had recently vacated. The marching and countermarching left the Armée d'Allemagne with its two wings separated by and joined by a thin cordon of Bavarian troops. On the same day the Austrian advance guard had beaten the Bavarians near Landshut and secured a good crossing place over the Isar by evening. Charles planned to destroy Davout's and Lefebvre's isolated corps in a double-pincer manoeuvre. Napoleon arrived in Donauwörth on 17 April and took command from Berthier. When Napoleon realised that many of the Austrian forces had crossed the Isar and were marching towards the Danube, he insisted that the entire French army deploy behind the Ilm River in a bataillon carré in 48 hours. His orders were unrealistic because he underestimated the number of Austrian troops that were heading for Davout; Napoleon believed Charles only had a single corps over the Isar, but the Austrians had five corps moving towards Regensburg, totalling 80,000 men. Landshut Maneuver Davout anticipated facing overwhelming forces and withdrew most of his forces, leaving 2,000 men from Regensburg. The northbound Austrian columns in the Kelheim–Abbach zone encountered four columns of Davout's men heading west towards Neustadt in the early hours of 19 April. The Austrian attacks were slow, uncoordinated, and easily repulsed by the experienced French III Corps. Napoleon knew there was fighting in Davout's sector and devised