[rnabel]

There is a great part in “Now it can be told”[0] which discusses the counter intelligence efforts of the Manhattan Project - specifically when trying to figure out how far along Nazi development was. The upshot is: approximately nowhere, IIRC the test reactors never fully worked.

However, in the process of finding out how much progress was made, they talked to multiple French nuclear scientists in newly liberated Paris. One of the scientists figured out that the US had working atomic weapons. This guy then managed to play this card to such effect (eg by suggesting that he may talk to the Soviets) that he was able to extract a lot of concessions from the US.

Now we have a nuclear armed France which IIRC was rapidly accelerated by tech transfer negotiated as a result. Pretty wild.

[0] https://www.goodreads.com/book/show/170428.Now_It_Can_Be_Tol...