Turnaround is fair-play. The Soviets acquired the F-5 (likely) from Vietnam, who captured them when South Vietnam fell. The U.S. has had a few Soviet aircraft operated in secret for quite awhile also. There's a display of captured East Bloc equipment at Nellis AFB, NV that has been there for decades, dubbed "The Petting Zoo." It has aircraft, tanks, APC's, SAM missile systems, etc., where visitors are allowed to climb on and in.