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Infantry, including missile troops, would typically be employed at the outset of the battle to break open infantry formations. Cavalry attempted to defeat the enemy cavalry.
Once one side coaxed their opposing infantry into breaking formation, the cavalry would be deployed in attempt to exploit the loss of cohesion in the opposing infantry lines and begin slaying the infantrymen from horse top.
Once a break in the lines was exploited, the cavalry became instrumental to victory - causing further breakage in the lines and wreaking havoc amongst the infantrymen, as it is much easier to kill a man from the top of a horse than to stand on the ground and face a half-ton destrier carrying an armed knight.
Until a significant break in the enemy infantry lines arose, the cavalry could not be used to much effect against infantry since horses are not easily harried into a wall of pikemen. Pure infantry conflicts would be drawn-out affairs.
A hasty retreat could cause greater casualties than an organized withdrawal, because the fast cavalry of the winning side's rearguard would intercept the fleeing enemy while their infantry continued their attack.
In most medieval battles, more soldiers were killed during the retreat than in battle, since mounted knights could quickly and easily dispatch the archers and infantry who were no longer protected by a line of pikes as they had been during the previous fighting.