Complex sentence: These troops were recruited with backgrounds in the Army, Navy, Air Force, and the Marines, and went to an Officer Candidate School to be trained and tested on their leadership, engineering, and psychology as well as their mental stability.
Split into simpler sentences: These troops were recruited with backgrounds in the Army, Navy, Air Force, and the Marines. The troops recruited went to an Officer Candidate School, where they were tested for their aptitude towards leadership, engineering, and psychology as well as their mental stability.