Complex sentence: English returned to the U.S. in December 1953 and was assigned to the Marine Corps Recruit Depot in San Diego once more until June 1957, while there he was promoted to full colonel in May 1954.
Split into simpler sentences: English returned to the United States in December 1953 and was assigned to the Marine Corps Recruit Depot in San Diego once more until June 1957. While there he was promoted to full colonel in May 1954.