Complex sentence: Edmonds had shown that the attack on Messines ridge was a limited attack, intended to advance and that the plan incorporated progressive elements like those used at Arras on 9 April, particularly an emphasis on counter-battery fire and a carefully controlled creeping barrage.
Split into simpler sentences: Edmonds had shown that the attack on Messines ridge was not an attempt at a strategic breakthrough and was limited to an advance. The plan incorporated progressive elements like those used at Arras on 9 April, particularly an emphasis on counter-battery fire and a carefully controlled creeping barrage.