Tough question... I would point to three incidents that led to the fall of Hitler. I guess you could say that the one that came first was the beginning of his fall.
To me, there are three major points that lead to Hitler's fall. They are:
- The failure to win the Battle of Britain. By not defeating Britain, Hitler allowed Britain to remain as a potential haven for enemies and staging ground for an invasion of Europe.
- Invading the Soviet Union. This took too many resources and ended up depleting them without any benefits.
- Declaring war on the US. Hitler didn't need to do this. When he did, he made an enemy of a country with huge resources. This ended up dooming him.