 Mack believed that the French would not violate Prussian territory, but when he heard that Bernadotte's I Corps had marched through Prussian Ansbach, he made the critical decision to stay and defend Ulm rather than retreat to the south, which would have offered a reasonable opportunity at saving the bulk of his forces.Napoleon had little accurate information about Mack's intentions or maneuvers; he knew that Kienmayer's Corps was sent to Ingolstadt east of the French positions, but his agents greatly exaggerated its size.On 5 October Napoleon ordered Ney to join Lannes, Soult, and Murat in concentrating and crossing the Danube at Donauwörth.