Complex sentence: Mileham (2000), p123 The King's losses accumulated and were more than 1,800 by the 3rd, with the Liverpool Irish, which acted in a supporting role, the heaviest: 18 officers and 304 other ranks.
Split into simpler sentences: Mileham (2000), p123 The King's losses accumulated; by the 3rd, the regiment's casualties were in excess of 1,800. The losses of the Liverpool Irish, which operated in a supporting role during the battle, were the heaviest; 18 officers and 304 other ranks.