Complex sentence: The decision to create Army service numbers was made in February 1918 with the first service numbers to be issued only to Army enlisted personnel; the Army officer corps was still relatively small, and the Navy was still maintaining ship rosters to keep track of its personnel.
Split into simpler sentences: The decision to create Army service numbers was made in February 1918 with the first service numbers to be issued only to Army enlisted personnel. The Army officer corps was still relatively small and the Navy was still maintaining ship rosters to keep track of its personnel.