Complex sentence: He served as an enlisted man in the United States Marine Corps 1942-1946 with twenty-eight months overseas and was awarded the Purple Heart Medal; he retired as a major in the United States Marine Corps Reserve.
Split into simpler sentences: He served as an enlisted man in the United States Marine Corps from 1942 to 1946, of which 28 months were spent overseas, and was awarded the Purple Heart Medal. He retired as a major in the United States Marine Corps Reserve.