Here is the thing , when I was going it was in the late 70,s , early 80,s . people went back and forth all the time , they even went deep into Mexico without paperwork all the time , BUT they did so at their own risk , they get in trouble down there , they had real problems because they didnt have the paperwork they were supposed to have , on top of what ever happened . As far as I know , you dont need a passport to go to US territories , like Puerto Rico , US Virgin Islands , etc . I went to PR in 2010 , it was like flying to another state DL only . But again , AZ , like most other states in the country , has a DL that is considered proper ID , so I never experienced any problems with air travel , and I spent 25 years with a company , where there was a 70% travel requirement . Went everywhere in the US practically , and some states I wouldnt pee on , if they were on fire , but thats another tale for another day lol . Had no idea that in this day and age , there were still a hand full of states that didnt furnish their residents with a proper DL . Its my opinion , that military personnel , and their immediate family ( wife / husband , kids ) be provided a US passport as part of the deal , from the get go .