Complex sentence: Soon after the battle, General French left the brigade to take command of the Third Division, II Corps, leaving Brooke in command of the brigade, a post he would hold off and on for the remainder of the war.
Split into simpler sentences: Soon after the battle, General French left the brigade to take command of the Third Division, II Corps. Colonel Brooke assumed command of the brigade, a post he would hold off and on for the remainder of the war.