Complex sentence: He served in the army from 1915 to 1919, and again from 1940 to 1946, achieving the rank of lieutenant colonel commanding the 2nd The Canadian Scottish Regiment (Princess Mary's).
Split into simpler sentences: He served in the army from 1915 to 1919. HE served again from 1940 to 1946, achieving the rank of lieutenant colonel commanding the 2nd The Canadian Scottish Regiment (Princess Mary's).