Bavaria sides secretly with Napoleon, and the Austrians, seventy-two thousand strong under Mack, prematurely invade while the Russians are still marching through Poland.

The Austrians expect the main battles of the war to take place in northern Italy, not Germany, and intend only to protect the Alps from French forces.

A popular but apocryphal legend has it that the Austrians used the Gregorian calendar, the Russians were still using the Julian calendar.

This meant that their dates did not correspond, and the Austrians were brought into conflict with the French before the Russians could come into line.

This simple but implausible explanation for the Russian army being far behind the Austrian is dismissed by scholar Frederick Kagan as "a bizarre myth". (Kagan, Frederick W. (2006). The End of the Old Order: Napoleon and Europe, 1801-1805. Cambridge, MA: Da Capo Press.)

In reality, the Austrians had expected that northern Italy, rather than Germany, would be the site of major battles.

On September 8, the army of Feldmarschall-Leutnant Karl Mack and Feldmarschall-Leutnant Archduke Ferdinand of Austria-Este crosses the Inn River and invades the Electorate of Bavaria.

Mack plans to establish eighty-eight battalions and one hundred and forty-eight squadrons on the Lech River near Augsburg by the end of October.

Though called upon to join Austria against France, Elector Maximilian IV Joseph of Bavaria instead withdraws his army north to the Main River in accordance with his secret alliance with France.