That proposal is not realistic. The U.S. had a very small number of nuclear weapons available. If I remember correctly, the number remained in the single digits

That proposal is not realistic. The U.S. had a very small number of nuclear weapons available. If I remember correctly, the number remained in the single digits for several years after 1945. To "conquer the USSR" would have required thousands of nuclear weapons, and they were not available nor was it possible to make that many. And regarding a non-nuclear war, the U.S. was pretty tired of fighting by the end of WW2.