 On 21 July, Dempsey started to secure his gains by substituting infantry for armour.Tactically, the Germans contained the offensive, holding many of their main positions and preventing an Allied breakthrough but they had been startled by the weight of the attack and preliminary aerial bombardment.It was clear that any defensive system less than deep could be overwhelmed at a stroke and the Germans could afford to man their defences in such depth only in the sector south of Caen.