The most sustained attempt to understand the nature of war, Carl von Clausewitz's On War, posthumously published in 1832, opens with a chapter entitled “What is war?” It immediately proceeds to a normative definition. Having described war as a duel, albeit on a larger scale, Clausewitz (1780-1831) concludes with a sentence which in most editions of the text is italicized: “War is thus an act of force to compel our enemy to do our will.”

For Clausewitz, therefore, the central elements of war are reciprocity and the use of force. It takes at least two to wage a war. The one-sided application of violence is not war, and the coercion of another without the use of force is also not war. In practice there may be qualifications to these norms. NATO's attack on Serbia during the Kosovo campaign in 1999 was, to all intents and purposes, a one-sided use of force, with minimal - if any - reciprocity, and the Cold War was waged by threatening the use of force, not by its actual employment (and that may be a very good reason for concluding that it was not in fact a war).

Significantly, nothing in this characterization of war is “Clausewitzian” in the sense used by contemporary journalism. So used, the epithet refers to a view of war as an instrument of policy, a view which refers to war's potential utility, not to its nature. Of course, if a state has recourse to war, its reasons can be called political. That is true even when the decision to fight is more instinctive than deliberative - for example, a response to invasion - and the war not one of choice but of survival.