The major reason napoleon’s invasion of russia failed was because the army was too small. the french lost faith in their leader. the russians chose to retreat. the army was unprepared for the weather.

The correct option is: "the army was unprepared for the weather."

The Napoleonic Invasion of, also called invasion to the Russian Empire led by Napoleon in 1812 was a turning point in the course of the Napoleonic Wars. The campaign reduced French and allied invasion forces to less than twenty percent of their initial capacity.

On the embers of a city in ruins, without having received the Russian capitulation, and with a new Russian maneuver forcing him out of Moscow, Napoleon ordered to retreat. The burnt-out city was tactically useless, aside from the fact that the nearness of winter made it impossible to keep close to 100,000 French soldiers surviving in the ruins of Moscow, for which Napoleon began the long retreat on October 19, after only six weeks of occupation . At the battle of Maloyaroslávets, Kutúzov was finally able to force the French to use the same road from Smolensk, which had earlier taken them east, to return to the west. At the time, they continued to block the southern flank to prevent the French from leaving this route.