Military in this sense is an American noun adaptation of an English adjective. To be used by Americans only.
I have never forgotten the opening sentence of our first GST lecture.
"You are now serving in Her Majesty's Royal Air Force. You are not and never will be in the RAFF! Understand?"
... the good old days when I was a Halton Apprentice and we got a free haircut every week
"Fagash Lil' "would give you a nice 'Boston' for going on leave if you slipped her two-bob.