Historian Peter Barton concentrates on the second phase of the battle, from the middle of July to the middle of September 1916. Barton reveals how an Allied advance on objectives which should have lasted days and weeks took months and resulted in yet more carnage on the Somme battlefield. Whilst British tactics were unvarying, that of their German enemy evolved using the changing landscape of the battlefield. The slowness of the Allied advance during this period Barton explains by the emergence of new German tactics - defence in depth.