Complex sentence: Much of the opening artillery barrage was off-target or not concentrated enough to have any impact, but the specialised armour worked well except on Omaha, providing close artillery support for the troops as they disembarked onto the beaches.
Split into simpler sentences: Much of the opening artillery barrage was off-target or not concentrated enough to have any impact. But the specialised armour worked well (except on Omaha), providing close artillery support for the troops as they disembarked onto the beaches.