 While Murat was making demonstrations across the Black Forest, other French forces would then invade the German heartland and swing towards the southeast by capturing Augsburg, a move that was supposed to isolate Mack and interrupt the Austrian lines of communication.On 22 September Mack decided to hold the Iller line anchored on Ulm.In the last three days of September, the French began the furious marches that would place them at the Austrian rear.