Robert Oppenheimer’s father was German immigrant.
There have been several foreign or foreign-born top scientists working on or making great contributions to the Manhattan Project - including Albert Einstein (indirectly, to an extent) Enrico Fremi, Emilio Segrè, Niels Bohr, (János) John von Neumann, (Jenó) Eugene Wigner, Edward Teller, Stanislaw Ulam, Josef Rotblat, Leo Szilárd, Hans Albrecht Bethe, just to name a few.
Of course, there was also Klaus Fuchs, but he was a Communist first, German-born Brit second.
The atomic bomb project in Germany, headed by brilliant German physicist Werner Heisenberg (of Heisenberg Uncertainty Principle fame), was not far along because Heisenberg “miscalculated” the amount of uranium needed to make the bomb by orders of magnitude - thus setting their reactor effort back several years, and to the U.S. nuclear program benefit when more than two tons of the German uranium stockpile was taken to Britain and then moved to the U.S.
If you ever visit the German state of Baden-Württemburg, there’s a little town in a deep valley south of Stuttgart and east of the Black Forest by the name of Haigerloch, which is worth a visit.
A cliff rises from near the river to a church and former nobleman’s residence at the top. At the base of this cliff is a secluded cave, enlarged during the Middle Ages to make a wine cellar.
This was the scene of the last attempts during the war by the German scientists to make a nuclear reactor. They dug a pit at the rear of the cave to hold several kiloliters of heavy water, and a 2-meter circular lid from which were suspended few dozen chains, each one a ‘necklace’ of small Uranium cubes.
They would lower the assembly into the tank of heavy water and measure the resulting reaction rates. They had not quite achieved criticality by the time the war ended, and they were rounded up by Allied forces. Some sources say that this was as far as they got towards a bomb; they were essentially at the stage the US was at in 1942, but with no resources to proceed further.
One of the reasons that work was relocated from Berlin to Haigerloch was that the scientists could sense Germany’s impending defeat, and they wanted to be as far west as possible when that day came. The reason for this desire is fairly clear.
After the war, the Germans were allowed to develop nuclear energy for electric power, of which they were early adopters. This early postwar work was, IIRC, centered at the University of Karlsruhe, also in southwest Germany. Tübingen, not far from Haigerloch, is the site of a historic University with a Physics faculty that served as a home-away-from home for some of the project scientists.
Today. the cave is a small museum, the “Atomkeller.”