Clausewitz terms "friction" the "only concept that more or less corresponds to the factors that distinguish real war from war on paper." (119) Friction is caused mainly by the danger of war, by war's demanding physical efforts, and by the presence of unclear information or the fog of war.
First, the intrinsically dangerous nature of war means that in an atmosphere of blood, bullets,and bombs, "the light of reason is refracted in a manner quite different from that which is normal in academic speculation." (113) Only the exceptional soldier keeps his incisive judgment intact during the heat of battle.
Second, physical effort in war also produces friction: "If no one had the right to give his views on military operations except when he is frozen, or faint from heat and thirst, or depressed from privation and fatigue, objective and accurate views would be even rarer than they are." (115) Clausewitz hence reminds strategists not to forget the immense effect of physical effort upon the soldiers engaging in combat.
Ambiguous information in war is yet a third element which Clausewitz says distinguishes real war from war in theory. Although strategists should gauge plans by probabilities, it is sometimes impossible to do so during war, when most intelligence is indeterminate:
"...[A] general in time of war is constantly bombarded by reports both true and false; by errors arising from fear or negligence or hastiness; by disobedience born of right or wrong interpretations, of ill will, of a proper or mistaken sense of duty, of laziness, or of exhaustion; and by accidents that nobody could have foreseen. In short, he is exposed to countless impressions, most of them disturbing, few of them encouraging...." (193)To offset the friction of war which results inevitably from human frailty, Clausewitz advocates pushing ahead with all one's might:
"Perseverance in the chosen course is the essential counter-weight, provided that no compelling reasons intervene to the contrary. Moreover, there is hardly a worthwhile enterprise in war whose execution does not call for infinite effort, trouble, and privation; and as man under pressure tends to give in to physical and intellectual weakness, only great strength of will can lead to the objective. It is steadfastness that will earn the admiration of the world and of posterity." (193)