This is politically incorrect, but the answer is: Germany. They were defeated because they were simply out-manned and outgunned. And, failures of their leadership (Hitler) sealed their defeat.
They thought they could ignore the lesson Napoleon was taught in Russia. But if they could have held off the Allies for another six months to a year, they could have used their atomic bombs that were almost ready to get the East Coast, maybe to the mid-west of the U.S. from Norway. (Yes, they were first in that as well as rocketry and jet aircraft.)
So now we are learning Spanish and Chinese instead of German. :-)