Blitz II is a massive World War II total conversion for C&C Generals: Zero Hour. You can fight as the Allies, Axis or the Soviets in the biggest World War II mod in existence.
I love how this mod gives a nice world war 2 feeling plus it has the complexity of the original blitzkrieg 2 game. I only rate this down because the instructions were difficult to follow and miss leading, as some steps such as uninstalling certain editions of the game being modded were not required.