America
The other powers before the war had been France, Britain, Italy, the USSR, and to a certain extent, Japan. France was occupied for most of WWII, Britain spent a lot of money and manpower on it, besides getting bombed pretty heavily, and Italy for obvious reasons was going to be dominated by the Allies for awhile. Japan was out due to being bombed by the US and because after the war, the US reshaped their economy and government.