Question

What was the major reason for Napoleon's defeat during his invasion of Russia in 1812?

Answer 1

The napoleons army was comprised of a dozen nations and a good sized larger. now i don't know what the actually plan was, but what did happen to was a poicy of attack and scorched earth. 

napoleons army couldn't sustain itself without a pillage (the towns were evacuated and burned beforehand) and in one notable instance of Moscow. 
The army couldn't not face Russia counter offensive at Moscow and they had to retreat thus they were defeated.