My mother (age 93) tells a story that echoes Magee's. someone she knew -- it may have been an older brother of a girl in school -- joined the RCAF in 1939. It seems most, if not all, Americans who did this chose the air forces, as had the ones who did the same in WWI before 1917. Anyone who joined the forces of a belligerent in a war in which the US was neutral violated the Neutrality Acts and their US citizenship was suspended. After the US joined the war, they wanted those experienced pilots back. The carrot was a commission in the Army Air Corps; the stick was having their US citizenship revoked. I met a gentleman at the Farnborough Air Show in 1992 who told me he had been quite happy as a Yank in the RAF, but he didn't want to lose his US citizenship, so he came back. But my mother's friend said, "You suspended my citizenship when I left, and you'll revoke it if I don't come back? Revoke away!" and became a Canadian citizen, which he remained to the end of his life.