Napoleon's Russian campaign of 1812 ended in catastrophe, costing the French around 500,000 men and marking the beginning of his downfall. Several interlocking factors caused the failure.
Russia's enormous size strained French supply lines. The army could not transport enough provisions and was forced to forage, alienating locals and failing to sustain troops.
The Grande Armée was ill-equipped for the brutal cold. Thousands died from exposure, frostbite, and starvation during the retreat.
The Russian army, larger and accustomed to winter warfare, employed a scorched-earth policy and avoided decisive battles until Borodino, then harassed the retreating French.
He underestimated Russian resolve, invaded too late in the season, and failed to plan for winter quarters.
Detached corps struggled to support each other, leading to missed opportunities and piecemeal defeats.
The disaster shattered Napoleon's aura of invincibility, encouraged his enemies, and directly led to his abdication in 1814.