In late September and early October 1805, Napoleon carried out a gigantic envelopment of the Austrian army in Bavaria led by Karl Mack von Lieberich. While the Austrian army lay near Ulm, south of the Danube River, the French army marched west on the north side of the river. Then Napoleon's troops crossed the river east of Ulm, cutting the Austrian retreat route to Vienna. Finally waking up to his danger, Mack tried to break out on the north side of the river, but a lone French division blocked his first attempt.