By traitors and spies, I'm referring to people sympathetic to the Axis cause at the time. I know that America put Japanese-American citizens in internment camps during the war, but I could imagine the conflict in Europe was much harder to discern.
Were there people within the Allied militaries(USA and UK mainly) that sympathised with the Nazi cause? Since the war was partly one of ideology, what steps were taken to ensure that no sympathisers or outright spies found their way into the ranks? Were people with known nazi/fascist sympathies rounded up in the same way as Japanese-Americans? What steps were taken for those they didn't know about?
As a related question, were there any Americans of descent from countries like Italy and Germany that left to join the war on the side of the Axis, feeling it was their duty to aid the struggle of their ancestral homelands? The American-German POW in Band of Brothers come to mind, who claims his family left to aid the struggle of national socialism.
I have watched a documentary about fascism in the US, it said that many nazi sympathisers were put in camps during the war, as you said like the japanese-americans. The irony is that some people of german origin who had no link with extremists movements were put in those camps and ended with more radical ideas after they lived with american nazis for years.
Well, Klaus Fuchs was spying for the Soviet Union while he helped in the Manhattan Project. He was a key reason the Soviets A) weren't very surprised by Hiroshima/Nagasaki and B) were able to build their own A bomb so quickly after WWII.