After World War II, which nation emerged as the strongest of the new superpowers?
America. Later on, you could make a strong argument that it could be the USSR.
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.
Add to that the fact that the US had the bomb, and the US was the most powerful country. The USSR, at this time, wasn't as powerful for a few reasons: a.) they wouldn't have the bomb until 1949, b.) a lot of Stalinist policies didn't work that well, and c.) people didn't like them very much. Even countries that didn't declare themselves the enemy of any and all Communism like the US did were against their expansionism and colorful history of human rights abuses, among other things.