Complex sentence: He served in the Twelfth Infantry of the New York National Guard (1889-1903), and in the Spanish-American War as captain of the Twelfth Regiment of New York Volunteers.
Split into simpler sentences: He served in the Twelfth Infantry of the New York National Guard 1889-1903. He also served in the war with Spain as captain of the Twelfth Regiment, New York Volunteers.