Lead up to the attack of Napoleon into Russia:

Russian armies under General Mikhail Kutuzov and Alexander I, emperor of Russia, reinforced the Austrians, but Napoleon crushed the combined Austro-Russian forces in the Battle of Austerlitz, sometimes known as the Battle of the Three Emperors.

Austria again capitulated, signing the Treaty of Pressburg on December 26, 1805.

Among the terms of this treaty was the concession by Austria to France of territory in northern Italy and to Bavaria of territory in Austria itself; in addition, Austria recognized the duchies of Württemberg and Baden as kingdoms.

Bottom line was that Napoleon felt threatened by the size of the Russian Bear.