They win the war of the 2nd, 3rd, 4th, and 5th coalitions. For the first time, they find that things are going fine. All they have to do is nothing. Pull back and consolidate. Maybe refine some things, tinker a bit. But it's not enough. It will never be enough. So they invade Russia.
In his life, Napoleon had success after success after success. Every step of the way, critics told him that he'd fail. But he did it anyway, and won 38 battles. 17 at near impossible odds.
How can someone like him be satisfied with sitting at home, ruling France? How could he be satisfied with puttering around in the countryside talking about his code with random farmers?
And so he decided to invade.
People warned him against invading. Armand de Caulaincourt staked his reputation on it, went with him on the campaign, and kept warning him. Warning him against wintering in Russia. He kept telling him to turn back. But Napoleon didn't listen.
They'd been wrong every single time. And he'd been right. Let them babble, his path was one set at a different level than theirs.
Except, that one time they were right.
It seems that when you are touched with greatness, you end up believing that you can do anything. And far more often than not, you can.
Right up until the moment you can't.