According to the passage, what was a major cause of both Napoleon’s and Hitler’s eventual downfalls?

The armies of both Napoleon and Hitler were severely weakened by their invasions of Russia, contributing to their eventual defeats. The reading doesn’t give information about their education (Neither Hitler nor Napoleon was well educated). Their strengths as in military strategists (They were both excellent military strategists) facilitated their conquests of most of Europe; it was not a cause of their downfalls. Both wanted to end British military superiority (and failed to achieve this goal), but trying to end British naval dominance (Both Hitler and Napoleon wanted to end British naval dominance) wasn’t a direct cause of their downfalls.