To take just one example, Britain's Sir Ernest Rutherford made important contributions to nuclear research regarding chain reactions. But he died before World War II began, so that doesn't "count" for this question. More to the point, perhaps Americans learned something from the British sabatageurs in Norway that disrupted the German heavy water experiments., because this took place during the war itself. Or did they learn anything else from British scientists during the war itself?
On the other hand, "German" scientists (that is the ones who stayed in Germany), would not have contributed to the U.S. nuclear research, at least not during the war. (Perhaps America benefited from Heisenberg's pre-war contributions.)
Finally, there were a large number of German refugee immigrants (14 of the top 28 German scientists according to Wikipedia) that did contribute to the U.S. atomic effort. I'm counting these as "Americans."
Using the above definitions, what contributions were made to U.S. atomic research by "non-Americans during the war?" Put another way, what would the U.S. have missed out on, or had to develop herself if atomic research were so sensitive that all governments put an embargo on it during the early 1940s?