I visited a B52 base in Georgia - Robbins AFB - in 1982, on NATO business to brief crews on MRR techniques. Flew in on civilian airlines via JFK, then the other NY airport (La Guardia?), Atlanta, Warner-Robbins. This must have been pre-visa days, although I was carrying a valid NTO that I do not recollect showing to anyone.
After a week of intensive training
of the crews (alright, took a couple of days and the remaining time was more usefully filled) , I flew back in a B52H (I think it was a H, tail gunner sat with the rest of the crew up front, but irrelevant to the point) to Fairford. So according to the US system I am still in the country, having overstayed by 24 years.
Subsequently managed to sneak back undetected to Las Vegas for a couple of flagex in the mid-80s, despite going through immigration channels at Washington airport; so maybe the system was not so tight back then, or was I just lucky?