Complex sentence: That day, Brooke ordered a counterattack to be spearheaded by two battalions, the 3rd Grenadier Guards and the 2nd North Staffordshires.
Split into simpler sentences: That day, Brooke ordered a counterattack. This was to be spearheaded by two battalions, the 3rd Grenadier Guards and the 2nd North Staffordshires.