2 things:

1) Stalin asked Britain and France for backing, not even support to attack Hitler after Hitler walked into the Sudetenland. If Russia had allied with them from the beginning, Hitler would be finished.
Instead, Stalin was turned down and made a nonggression pact with Hitler, who then attacked Poland, forcing Russia to go on the offensive on their own
2) If, when Hitler attacked France, they had fought back, Hitler would be finished.
They didn't, instead collaborating with him. France could easily have held off Hitler at the beginning of the war