Why was napoleon invasion of Russia considered a mistake?

Answer 1

the French invasion of Russia was a mistake because it achieved nothing for the French territory-wise, and Napoleon lost the cream of his army; When he invaded Russia, with French and Allied troops (majority were allied), he had one of the largest armies Europe had ever seen: The Grandé Armée numbered 690,000 men with other Armies making the total forces upwards of 800,000 under Napoleon (only 300,000 of those were French). At the end of the invasion, his forces numbered less than 70,000. His men had been whittled down by bloody battles, Guerilla warfare, but mostly nature: His troops had little food because the Russian's burnt the land as they retreated, and they were not equipped to deal with the freezing temperatures: they got as low as -30 degrees celsius. This massive loss of men meant that Napoleon had lost a lot of veterans not easily replaced, and made it very difficult for him to fight of the allied armies in later wars.

Answer 2

He could not conquer russia because the land was too extensive for his army

and he was not prepared for Russian winter.