[nostrademons]

The U.S. wasn't a (geopolitical) superpower before WW2, although it was definitely a strong rising economic power. Britain was considered the world's superpower, with other major Great Powers being the U.S, France, Germany, Italy, Russia, and Japan. The U.S. became a geopolitical superpower by virtue of being the most undamaged of the Great Powers after the war.

Similarly, it's likely that whichever industrialized high-tech region that's unaffected by WW3 would take on the role of the post-war superpower. I can't tell what that be - it could be a country like Brazil, Australia, New Zealand, Scandinavia, Kenya, or a region of an existing superpower like California, Ireland, or the Pearl River Delta - but it's likely that there will be some portion of the globe that is not destroyed and yet still retains enough technical knowledge and capital base to rebuild the rest of the world.