On June 24th, 1812, Napoleon led an Army of a half million men, his Grande Armee, across the Nieman River, into Russia. The army he led was composed of troops from almost all of Europe, including Poland, Prussia, Saxony. Austria, France, the Lowlands, Italy, and even Portugal.
The invasion was inevitable. Napoleon was increasingly frustrated by Russia's refusal to comply with his "Continental", anti-British economic blockade. Like Hitler, over a century later, he seemed to believe he needed to defeat Russia to force the British to sue for peace. And the Czar, Alexander, no longer in awe of Bonaparte, wanted war.
Napoleon's invasion was based on two basic calculations. First, he would bring the Russians to battle rapidly by driving on St. Petersburg. Second, his army would live off the land in doing so. Bonaparte was wrong on both counts.
The Czar and his government abandoned St. Petersburg for Moscow. Napoleon now faced a front three times as deep as he planned for. And the Emperor who once said, "ask anything of me but time", now found himself on a longer campaign than he'd contemplated as the Russian Army retreated before him, refusing to be drawn into battle, scorching the earth as they went.
No battle was actually fought until late summer [Borodino]. And although the Emperor won, he took losses he couldn't afford, lost more time he couldn't afford, and didn't reach Moscow until September. He stayed there, trying to reach a diplomatic solution with the Czar, who was more than willing to talk, and waste more time. That lasted until winter was on the doorstep and Moscow was in flames. At that point, Bonaparte decided to withdraw [retreat, actually]. The plan was to swing southeast into Ukraine, and retreat that way. But the Russian Army, for the second time, stood in Bonaparte's way and offered battle. This time, Napoleon declined, and retreated the way he came via Smolensk and Minsk. It was a critical mistake. The Grand Armee had already picked over whatever there was to pick over on that route on their way to Moscow. They had burned most of the lumber from huts, etc. for cooking on the way in. The cupboard was bare, the troops had no winter uniforms, it was the worst winter in 50 years, and Russian irregulars and Cossacks picked off stragglers and foragiung parties like flies.
By the time he reached Smolensk, Napoleon hopped in a sled, abandoned his Army to the command of his Marshals, and fled to Paris, to save his throne, and spin the disaster his Army was suffering through. By late winter, the French rearguard crossed the Nieman [Marshal Ney was the last soldier out]. They numbered less than 10% of the troops who had gone in. The Prussians changed sides in the middle of the campaign. But Napoleon's major loss were the horses. He lost over half a million. Neither his cavalry, nor his artillery ever fully recovered from those losses. And Napoleon without artillery and cavalry wasn't Napoleon.
He campaigned through 1813, lost Leignitz, the Battle of Nations; was forced back into France, where in 1814, with his Marshals telling him it was over, abdicated, leading to Elba, the return, the Hundred Days, and eventually, St. Helena.
So aside from the huge loss of life, the carnage, and the loss of his throne, what did Napoleon get for crossing the Nieman? One hell of an Overture. At least that beats a T-shirt.