If any single date could be chosen it would have to be the 19th of October 1812 when Napoleon began the Grand Armee retreated from Moscow, the inability of Napoleon to force the Russian army into a decisive battle earlier in the campaign meant that the Russian army was was able to harass the Grand Armee down to 27,000 effective fighting men by the time they crossed the Berezina, this was the first time Napoleon was ever decisively defeated in a campaign ruining the perception of French invincibility on the battlefield, thus when the Russian army started to counterattack into Europe they were joined by Prussia and Austria who had been French allies during 1812, this sixth coalition against Napoleon was finally able to succeed in defeating Napoleon at the battle of Leipzig in 1813, which gave impetus to the desertion of almost the entire Confederation of the Rhine and thus loss of Germany east of the Rhine, in 1814 a series of defensive battles by Napoleon delayed the advance on Paris but after it fell Napoléon surrendered to the forces of the Sixth coalition.

There are many myriad reasons for this chain of events including Napoleon running roughshod over many of his client states and allies that led to them deserting him as soon as the French army showed weakness, the Spanish ‘ulcer’ being a notable outcome of these actions wherein Napoleon attempted to set up his brother as King of Spain(a French ally at the time) while on his way to invade Portugal(a British ally), this led to the infamous ‘guerilla’(little war) with atrocities committed both by and against the occupying French soldiers which costed the Napoleonic Empire heavily in both currency(Napoleon stated that 300million Francs had been spent by January 1810) and manpower(French casualties are estimated at 300,000 for the entire Peninsula war against Spain, Portugal, and Great Britain)

The direct cause for the French invasion of Russia was Russian refusal to follow the continental system, wherein France and all its allied states were forbidden from trade with Great Britain. Angered by the Russian efforts to circumvent this economic warfare Napoleon prepared the Grand Armee to invade Russia planning to destroy the Russian army within a short distance of the border(this strategy was a central part of Napoleonic doctrine), however the Russian army had similarly concluded this would be his plan and retreated further into Russia, only attempting to make a Decisive stand near the village of Borodino at the approaches to Moscow, the battle resulted in stalemate and the Russian army subsequently abandoned Moscow, Napoleon then planned to winter in Moscow before attacking northwest towards St. Petersburg. Unfortunately for Napoleon the City of Moscow burned down after the French army occupied it(the causes of this fire are still disputed to this day but are likely to have been Sabotage) removing any winter quarters for his army, with no place to winter and Russian refusal to surrender Napoleon finally had to admit defeat due to a complete lack of supply for his Army.

Please note I am not an expert, and would happily have this reply removed as long as an actual expert responds. I would just like to offer a simplified answer to the original poster in the case that none is left by an expert.