There were leaks before the Manhattan project even concluded: https://en.wikipedia.org/wiki/Klaus_Fuchs https://en.wikipedia.org/wiki/Klaus_Fuchs Sure, non-US

There were leaks before the Manhattan project even concluded: https://en.wikipedia.org/wiki/Klaus_Fuchs https://en.wikipedia.org/wiki/Klaus_Fuchs Sure, non-US nuclear arms would have most certainly happpened either way, but exfiltration did presumably at least accelerate the timeline.