In the USMC in 1986, one of my men (I was Sgt in charge of the FLIR repair shop) got told his planned destination for Christmas leave was on the "no-travel list".
He got a funny look on his face and told the Captain "I'll have MY ambassador call YOUR president about that!".
The Captain managed to think enough to realize something was funny here (the thick brogue helped here, I think), and soon found out that Cpl Bray was indeed an Irish citizen (Eire, not N.I.), and the address on his leave request was his parents' house... and also his "home of record". It seems his older brother was a naturalized US citizen, and had sponsored him, which allowed him to join the USMC.
Regs required them to let him go "home" on leave, so he got an Intel briefing before leaving from a spook Major... who said afterwords that Blackie had told him more than he had told Blackie about who was doing what to whom, and where!