Start with the Manhattan project, and work backwards from there. Operation Paperclip may have been a failure, but that was after the war. Any modern electrical engineer's professional lexicon is dripping with the names of the titans of our field. I assume the same is true for physicists, because we electrical engineers stand on the shoulders of physicists every day.

But don't take my word for it: https://en.wikipedia.org/wiki/Einstein%E2%80%93Szil%C3%A1rd_...

This article describes the letter that Leo Szilard wrote to Albert Einstein, in 1937, which Einstein essentially put his existing scientific prowess behind as an appeal to expertise, and forwarded it on to Franklin Delano Roosevelt. In it, he enumerates some of the latest discoveries Enrico Fermi has been finding in Uranium. The letter simply supposes that if these scientists in the US are discovering the possibility to induce a chain reaction in Uranium, it is a safe bet to assume that the Germans were "peer reviewing", if you will. Another factor Einstein lays out is all of the known key Uranium deposits around the world: Czechoslovakia and The Congo, among others.

Einstein/Szilard have the foresight to deduce that the Germans stopping all public sales of Uranium in Czechoslovakia, along with the very short human network from those Czechoslovakian Uranium mines to the German leadership, is extremely compelling circumstantial evidence that Germany has already started their Manhattan Project, and have all the benefits of incumbency behind them.

How did the US close the loop faster than the Germans? We hired Hungarian refugees fleeing Europe, who ran absolute circles around the rest of the known universe in Mathematics. Any modern student of mathematics can easily recite their names:

http://slatestarcodex.com/2017/05/26/the-atomic-bomb-conside...

And then in its usual cosmic sense of humor, the universe brought Kurt Godel's incompleteness theorem back around the eternal golden braid, and brought us the Cold War.