Complex sentence: One of the aircraft based at Atsugi at least since 1957 was the U- 2 spy plane piloted by Gary Powers, which provoked an international incident when it was downed over the Soviet Union.
Split into simpler sentences: One of the aircraft based at Atsugi at least since 1957 was the U- 2 spy plane. This same plane was piloted by Gary Powers, which provoked an international incident when it was downed over the Soviet Union.