Complex sentence: In 1941, he received his Ph.D. in history from Columbia University and enlisted in the US Navy as an Ensign; he mustered out in 1946 holding the rank of Lieutenant Commander.
Split into simpler sentences: In 1941, he received his Ph.D. in history from Columbia University. In that year, he enrolled in the US Navy as an Ensign, and resigned in 1946 holding the rank of Lieutenant Commander.