Complex sentence: He was sent to the Marine Corps Recruit Depot Parris Island, South Carolina, where, after twelve weeks' intensive recruit training, he was promoted to the rank of Private First Class and became the push-up champion of his battalion.
Split into simpler sentences: He was sent to the Marine Corps Recruit Depot Parris Island, South Carolina, for twelve weeks of intensive recruit training. After recruit training, he was promoted to the rank of Private First Class and became the push-up champion of his battalion.