While critiquing northern and southern generalship in the American Civil War, the distinguished historian, T. Harry Williams, claimed that the war was not just a struggle of men and material but also “a war of ideas.” He was referring not only to the political or social ideologies then being contested between the northern and southern states but especially to the military theories and beliefs that guided the decisions of the leading generals on each side. Williams claimed that Union generals Ulysses S. Grant and William T. Sherman adjusted their ideas of warfare to meet the strategic and operational demands of the war, while Robert E. Lee failed to adapt from his limited, Jominian, prewar conceptions. In the last few decades, historians have begun to apply Williams' assertion to other wars, which were, for the officers who directed the combat operations, just as much wars of ideas.
When such ideas are widely agreed upon in an army and codified in some way, either formally or informally, they become military doctrine – the officially sanctioned ideas and methods that are to govern combat operations. These core beliefs should influence the army's force structure, training, armament, battle plans, and tactics. In turn, doctrine must remain in harmony with the changing conditions of the battlefield – particularly, developments in weaponry, limitations in training, logistical constraints, strength of the enemy force, and even terrain and climate.