On July 23, 1812, he launched his army across the border, despite the protestations of many of his Marshals. The Russian Campaign had begun, and it would turn out to be Napoleon's biggest blunder. Russia's great strategic depth already had a habit of swallowing armies, a fact many would-be conquerors learned the hard way. Napoleon, exceptional though he was in so many regards, proved that even military genius can do little in the face of the Russian winter and the resilience of its people.

From a purely military standpoint, much of the campaign seemed to be going in Napoleon's favor since he met with little opposition as he pushed forwards into the interior with his customary lightning speed, but gradually this lack of engagements became a hindrance more than a help; Napoleon needed to bring the Russians to battle if he was to defeat them. On September 7, 1812, he must have thought his prayers had finally been answered, as the Russians had decided to stand and fight almost at the very gates of Moscow. Through his looking glass, Napoleon's well-trained eye observed the weakness of his opponent's defensive position. French and allied infantry advanced, anticipating that their Russian foes would again break as they had done two months earlier at Smolensk. The cavalry and Imperial Guard stood ready to complete the rout, this time preventing any organized withdrawal, and laying open an unopposed march to seize Moscow. 

The Battle of Borodino, as it was later called, resulted in a combined casualty toll of over 75,000, a hideously long butcher's bill that represented the bloodiest single day of the Napoleonic Wars. The Russian army retreated and Napoleon was able to occupy Moscow, hoping this would persuade the tsar to sue for peace. However, even as his advance guard pushed into the city, the retreating Russians put the capital to the torch. The Russian army's retreat also ensured that it would live to fight another day, if necessary.