Complex sentence: Troops in the (second) line were withdrawn behind the ridge and by the end of May, the front battalions were being changed every two days instead of every five, due to the effect of the British bombardment.
Split into simpler sentences: Troops in the (second) line were withdrawn behind the ridge. By the end of May the front battalions were being changed every two days instead of every five, due to the effect of the British bombardment.