Complex sentence: He was attached to the infantry as a range-finder at the Battle of the Somme in 1916 and in 1917 he was promoted in the field to lieutenant and then served at the Battle of Cambrai.
Split into simpler sentences: He was attached to the infantry as a range-finder at the Battle of the Somme in 1916. In 1917 he was promoted in the field to lieutenant and served at the Battle of Cambrai.