Was invading Russia the main cause for Napoleon's downfall? Or was his empire already growing uneasy by this point? What's the latest into his ruling that he could have 'taken a chill pill' and stopped starting so many wars and maintained his power?
Just been doing some light reading of him lately, and he seems to have just gone around hitting a bunch of hornets nests and getting away with it, surely even if he tried for peace, the rest of Europe would have just used it as a chance to rebuild their forces and get some payback, right?
For lack of a better way of saying it, the invasion of Russia is not the main cause of Napoleon's Downfall. Due to how Napoleon conducted diplomacy with the defeated, the former enemy nations were very angry and annoyed with Napoleon.
If there is main cause for Napoleon's downfall, I would point to 1806 and the creation of the Continental System. Due to the need to deprive Britain of vital trade, Napoleon looked to restricting Europe from trading with Britain and thus hoping to starve Britain of the cash needed to fund the wars against Napoleon. As a result, Napoleon developed the Continental System in order to have all complying nations not trade with Britain, preventing Europe from having income from their own trade with Britain. This would make the nations made as they became poorer from lack of trade or would see money lost due to black market trade.
Further, the only two offensive wars that Napoleon conducted (the invasion of Russia and the Peninsular War), were direct enforcement of this policy that went horribly. The Peninsular War was a very massive drain on French manpower and logistics that it deprived very needed reinforcements and munitions. However, once Napoleon invaded Russia, it took time for the ball to get rolling but in 1813 many of Napoleon's former enemies rose up again out of the anger for Napoleon's restrictive economic policies and the requirements to provide soldiers and quartering for soldiers.
So, Napoleon's defeat was mainly caused by Napoleon.