The US literally beat the Nazis to developing fission technology, i.e. nukes (admittedly with a very international research community). It's quite clear just from that, that the US had plenty of strong scientists before they brought in Nazis/Nazi collaborators from overseas.

As a complete side note: I believe it's been speculated (by people who know much more about this than me) that Nazi research on nukes, among other things, was hampered by researchers like Heisenberg deliberately dragging their feet because they were forced to work on the projects but didn't believe in the cause. I'm not meaning to clear the name of any Nazi collaborators, but pointing out that not all scientists working under the Nazi regime were necessarily nazis.