When did that start? Lots of American men joined the RAF because they didn't want to wait for America to get into the war, and I can't believe they'd have done so if they'd had to forfeit their citizenship.
The US saw WW 11 looming...........it is believed that in 1940-41, Pres Roosevelt signed a secret order allowing US military personnel to resign their posts to join anti-axis military action.......preparatory to the US getting involved overseas.
And that continued. I personally knew a US high school teacher, at the high school I graduated from, who flew for the IAF in the 6 Day War in 1967. He maintained his US citizenship, thereafter...
the infoiwarrior