Emperor Napoleon Bonaparte had launched his 200,000-man Grand Army across the Rhine. This huge mass of maneuver wheeled to the south and crossed the Danube River to the east of (i.e., behind) General Karl Freiherr Mack von Leiberich's concentration at Ulm. Unaware of the force bearing down on him, Mack stayed in place as Napoleon's corps spread south across the Danube, slicing across his lines of communication with Vienna. In the Battle of Wertingen (8 October 1805) Imperial French forces led by Marshals Joachim Murat and Jean Lannes attacked a small Austrian corps commanded by Feldmarschall-Leutnant Franz Xaver von Auffenberg. This action, the first battle of the Ulm Campaign, resulted in a clear French victory. The Austrians were decimated, losing nearly their entire force, 1,000 to 2,000 of which were prisoners.