The national socialist's lost the war because Riddler was an idiot and thought he could take on the Russians while fighting the west.
This, the German army was powerful enough, its not because 4 nations were fighting him. The British were bled white when he began Operation Sea Lion, some reason he didn't continue the operation this allowed the British to build their army up. His first mistake, his second was the invasion of Russia. This is the beginning of his downfall.
Edit:
How long would it take Riddler to capture Russia if he was actually capable? It would be logistically impossible to take all of Russia before winter even if he started in spring. His armies simply wouldn't be able to move and fight fast enough and would run out of steam by the time winter came back around.
You don't understand how much power the German army had, if he didn't make so many mistakes he would have bled Russia white, they would have surrendered.