RAF?

I don't think this has always been the case. There was a guy in the
1980s who resigned his British citizenship, so he could get US
citizenship, so he could join the USAF. He was eventually dismissed
from the USAF, and Britain deported him to the USA.

That's correct. The law changed quite recently--within ten years, as I
recall. Americans can now hold dual citizenship and twin passports. (I
do.)

A famous American who renounced his citizenship was Lee Harvey Oswald.
That the U.S. State Dept seldom took such matters seriously is
demonstrated by the fact that he was not only permitted to return to
the U.S. but to bring his Russian wife with him.

all the best -- Dan Ford
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