The key error made by Napoleon during his invasion of Russia in 1812 was underestimating the harshness of the Russian winter, as well as the Scorched Earth policy employed by the Russians. Napoleon's Grande Armée advanced deep into Russia but faced severe supply issues and a lack of proper winter preparations. As the temperature plummeted, diseases, starvation, and attacks from retreating Russian forces decimated his army. By the end of the campaign, only a fraction of his troops returned, marking a significant turning point in the Napoleonic Wars and leading to his eventual downfall.