During 20 years as a USAF officer I never met another officer wearing the US flag who was not a US citizen. Lots of non-citizens are enlisted in the US forces as others have stated and I have known many. There are also lots of foreign officers who are in US units but are still members of their own country's forces. There are lots of exchange officers including ones from Canada. I have known Canadian Air Force pilots who were exchange officers with the USAF. They flew the same planes and did the same work as the USAF pilots. In fact we had a Canadian writing our basic flying reg for quite awhile. There are many more that are not exchange officers but are part of a training agreement. All NATO members and many others can come to the academies, UPT and professional development courses. When I went to UPT we had Kuwaiti, Saudi and various Central American officers attending. Again they were not "in" the USAF but doing the same training.