The invasion had to be delayed weeks because the wehrmacht had to help Mussolini and his weak willed troops. It was a necessary gamble hitler nearly won and likely lost because of the delayed invasion. Hitler's greatest mistake was probably honoring his relationship with japan after pearl harbor and declaring war on the US, this is all too easy to say after the fact though. The russian winter meme is real though, Hitler's generals should jave been more forceful about timing.