A little before 16:00, Ney noted an apparent exodus from Wellington’s centre. He mistook the movement of casualties to the rear for the beginnings of a retreat, and sought to exploit it. Following the defeat of d’Erlon’s Corps, Ney had few infantry reserves left, as most of the infantry been committed either to the futile Hougoumont attack or to the defence of the French right. Ney therefore tried to break Wellington’s centre with cavalry alone. Wellington’s army responded by forming squares (hollow box-formations four ranks deep).
#The_French_cavalry_attack
From the movie Waterloo.