An Art of War Was Needed
[By the thirteenth century C.E.,] crossbows and pikes had to be supplemented by cavalry for flank protection and the pursuit of a vanquished foe. This obviously made war far more complicated than it had been when a headlong charge by a group of knights dominated the battlefields of Europe. Simple personal prowess, replicated within knightly families across the generations, was no longer enough to win battles or maintain social dominion. Instead, an art of war was needed. Someone had to be able to coordinate pikes, crossbows, and cavalry. Infantrymen needed training to assure steadiness in the ranks, for, were their formation to break apart, individual pikemen would find themselves at the mercy of charging knights; and the time required to cock a crossbow meant that archers, too, became vulnerable each time they discharged their weapons, unless some field fortification or an unbroken array of friendly pikes could protect them until they were ready to shoot again.
— The Pursuit of Power, p. 68