Complex sentence: Oswald was a former U.S. Marine who defected to the Soviet Union in October 1959 where he lived until June 1962, at which time he returned to the United States.
Split into simpler sentences: Oswald was a former U.S. Marine who defected to the Soviet Union in October 1959. He lived in the Soviet Union until June 1962, at which time he returned to the United States.