This article presents an analytic critique of the predominant revisionist theoretical paradigm of just war (henceforth: revisionism). This is accomplished by means of a precise description and explanation of the practicability problem that confronts it, namely that soldiers that revisionism would deem “unjust” are bound to fail to fulfil the duties that revisionism imposes on them, because these duties are overdemanding. The article locates the origin of the practicability problem in revisionism’s overidealized conception of a soldier as an individual rational agent analogous to the aggressor or defender in a case of lethal self-defense, who is capable of reflecting on the morality of his status in war and of the killing he performs and thus of recognizing his revisionist duties. Revisionism, however, ignores the following fact: Killing in war is not a natural human behavior. This is why training soldiers to kill is—and arguably always has been—a necessity for the existence of war and the killing that occurs in it. Moreover, this training involves a certain level of moral desensitization to violence whose goal is to prevent soldiers from thoroughly reflecting on the morality of the killing they engage in. Hence, war and killing can only exist if soldiers are trained in such a way that they do not reflect on whether they could be addressees of revisionist duties in the first place. This means that military training is a “constitutive condition” of soldiers and war, which is why it cannot excuse their noncompliance with revisionist duties, thus making these duties categorically overdemanding. The argument here draws on the paradigmatic example of modern US military killing conditioning (MC), but embeds it into a broader military-historical perspective that describes how soldiers have always needed to be mentally and morally influenced in order to enable war and killing. The article’s explanation of revisionism’s practicability problem has a constructive consequence for future theory-building in the ethics of war: It implies that a potentially revised ethical theory of war must necessarily analyze the institutions that allocate belligerent resources, if it aims to morally assess battlefield behavior in a practicable manner.