Most of them were not naturalized until the early 40s, and two of the most important, Enrico Fermi and Leo Szilard, were not naturalized until well after the Manhattan Project was underway. They were no more or less American than Mr. Mascheroni.
They were Americans because they lived here legally, that makes them Americans, the fact that they became naturalized later only supports my point that they were, indeed, Americans. The original commenter was talking about Germans being the scientist who developed the A-bomb, he/she was implying that we brought Germans over to develop it. This is simply not true, and if you can’t see that, tough sh**. He was mixing his nuclear scientist with the rocket scientist who came here after the war.