Complex sentence: The first step is the Battle of Menin Road, on 20 September, the 1st and 2nd Divisions are supported by the field artillery of their own and the two reserve divisions, and 46½ batteries of heavy guns.
Split into simpler sentences: The first step is the Battle of Menin Road, on 20 September. The 1st and 2nd Divisions are supported by the field artillery of their own and the two reserve divisions, and 46½ batteries of heavy guns.