Complex sentence: Understrength due to the small numbers of replacements being sent from Britain and in incomplete defences, the Fifth Army was forced back quickly after the first two days, as the Germans advanced under a heavy bombardment of high explosives and gas.
Split into simpler sentences: Understrength due to the small numbers of replacements being sent from Britain. In unfinished defences, the Fifth Army was forced back quickly after the first two days, as the Germans advanced under a heavy bombardment of high explosives and gas.