Complex sentence: In September 1968 he was assigned to the Chief of Naval Operations and headed the Aviation Plans Branch until February 1970 and then headed the Strategic Plans Branch until June 1971, for this service he was awarded the Legion of Merit.
Split into simpler sentences: In September 1968 he was assigned to the Chief of Naval Operations and headed the Aviation Plans Branch until February 1970. He then headed the Strategic Plans Branch until June 1971, a service for which he was awarded the Legion of Merit.