From Wikipedia

Napoleon - The Summit Of Ambition

The French invasion of Russia of 1812 was a turning point in the Napoleonic Wars. The campaign reduced the French and allied invasion forces to a tiny fraction of their initial strength.
At the time of the invasion, Napoleon was at the height of his power with virtually all of continental Europe either under his direct control or held by countries defeated by his empire and under treaties favorable for France. No European power on the continent dared move against him. The 1809 Austrian war treaty had a clause removing Western Galicia from Austria and annexing it to the Grand Duchy of Warsaw. Russia viewed this as against its interests and as a potent launching point for an invasion of Russia. Tsar Alexander found Russia in an economic bind as his country had little in the way of manufacturing and being rich in raw materials yet being part of Napoleon's continental system denied it the trade that was its lifeblood for both money and manufactured goods. Russia's withdrawal from the system was a further incentive to Napoleon to force a decision.