Napoleon's defeat began with a decision to invade Russia in summer

When Napoleon crossed into Russia in June 1812 with 600,000 troops, he expected a decisive battle within weeks. The Russian army refused to engage, instead retreating and burning villages and crops as they went — a scorched earth strategy. French supply lines stretched impossibly thin. Napoleon reached Moscow in September to find it abandoned and burning. By the time the Grand Armée retreated in November, temperatures had fallen to -30°C and starvation was endemic. Only 100,000 men returned to France. The campaign shattered the myth of Napoleonic invincibility.

Source: Lieven, Russia Against Napoleon (2009)