Modern wars are seldom fought without hatred between nations; this serves as a more or less substitute for the hatred between individuals. Even when there is no natural hatred and no animosity to start with, the fighting itself will stir up hostile feelings: violence committed on superior orders will stir up the desire for revenge and retaliation against the perpetrator rather than against the powers that ordered the action. It is only human (or animal, if you like), but it is a fact.

[Der Nationalhaß, an dem es auch bei unseren Kriegen selten fehlt, vertritt bei dem einzelnen gegen den einzelnen mehr oder weniger stark die individuelle Feindschaft. Wo aber auch dieser fehlt und anfangs keine Erbitterung war, entzündet sich das feindselige Gefühl an dem Kampfe selbst, denn eine Gewaltsamkeit, die jemand auf höhere Weisung an uns verübt, wird uns zur Vergeltung und Rache gegen ihn entflammen, früher noch, ehe wir es gegen die höhere Gewalt sein werden, die ihm gebietet, so zu handeln. Dies ist menschlich oder auch tierisch, wenn man will, aber es ist so.]

Karl von Clausewitz (1780-1831) Prussian soldier, historian, military theorist

On War [Vom Kriege], Book 2, ch. 2 “On the Theory of War [Über die Theorie des Krieges],” § 17 (2.2.17) (1832) [tr. Howard & Paret (1976)]
    (Source)

(Source (German)). Alternate translations:

National hatred, which is seldom wanting in our wars, is a substitute for personal hostility in the breast of individual opposed to individual. But where this also is wanting, and at first no animosity of feeling subsisted, a hostile feeling is kindled by the combat itself; for an act of violence which any one commits upon us by order of his superior, will excite in us a desire to retaliate and be revenged on him, sooner than on the superior power at whose command the act was done. This is human, or animal if we will; still it is so.
[tr. Graham (1873)]

National hatred, which is seldom lacking in our wars, becomes a more or less powerful substitute for personal hostility of individual against individual. But where this also is wanting, and, at first, no animosity existed, a hostile feeling is kindled by the combat itself. An act of violence which anyone commits upon us by order of his superior will excite in us the desire to retaliate and be revenged on him sooner than on the superior power at whose command the act was done. This is human -- animal, if you will -- but it is a fact.
[tr. Jolles (1943)]