Nazi Germany had an aggressive nuclear weapons program that was months away from producing a working fission bomb that could have been mounted on a V-2 rocket and used to devastating effect anywhere on the Eurpoean continent, and, soon after, anywhere in the world. In fact, the U.S. nuclear weapons program was heavily staffed by German Jewish refugees. If Hitler wouldn't have scared them away, and into our hands, he would have ended the war with nuclear blasts. Not us.
The U.S. has never had a magic ability to keep other countries from developing nuclear weapons. We simply got there first, and largely due to dumb luck.
Blame it on Albert Einstein (a German Jew), Enrico Fermi, J. Robert Oppenheimer (Einstein, Fermi and Oppenheimer were all alumni of Göttingen University in Germany) and the other brilliant nuclear physicists of the twentieth century, if you feel the need to lay blame on someone, but don't blame the United States for surviving the nuclear arms race.
Germany was working on development of nukes. Japan was working on development of nukes. Russia was working on development of nukes. The US was working on development of nukes.
Japan's nuke facility was in present-day N Kor, BTW.