The combat of Wertingen (8 October 1805) was the first significant fighting of the Ulm campaign, and saw part of the French advance guard defeat an Austrian column ten miles to the south of the Danube.
Napoleon's plan at the start of the War of the Third Coalition was to advance rapidly into Germany and defeat the Austrians before their Russian allies could arrive on the scene. While the Austrians, under General Mack, advanced along the Danube to Ulm, Napoleon crossed the Rhine further north and cut across Germany to reach the Danube east of the Austrian positions. A combination of excellent security and a good cavalry screen meant that the Austrians didn’t discover what was going on until it was too late.
After crossing the Danube Napoleon split his army, with one part advancing to the east of the River Lech and the other half to its west. The first significant fighting on the War of the Third Coalition came when the advance guard to the west of the Lech ran into an Austrian force of nine infantry battalions and one cavalry squadron under General Auffenberg that was heading west to join Mack at Ulm. The first French force consisted of a large number of cavalry commanded by Murat and Lannes. The Austrians formed square and managed to hold off the French cavalry but when Oudinot arrived with some infantry the Austrians were forced to retreat. The French captured between 2,000 and 3,000 prisoners out of an original force of 8,000. The French also gained valuable intelligence from their prisoners.
Over the next few days the French consolidated their position south of the Danube. The north bank was left dangerously unguarded, and the Austrians came close to escaping on 11 October (battle of Albeck). A second breakout attempt was frustrated on 14 October (battle of Elchingen) and after that it was just a matter of time before Mack surrendered at Ulm.