In 1975 Colin Powell entered the National War College in Washington. Once there, Colonel Powell (as he was then), a veteran of two tours in Vietnam, read Carl von Clausewitz’s On war for the first time. He was bowled over. On war was, Powell recalled, ‘like a beam of light from the past, still illuminating present-day military quandaries’. What particularly impressed him was Clausewitz’s view that the military itself formed only ‘one leg in a triad’, the other two elements of which were the government and the people. All three had to be engaged for war to be sustainable; in the Vietnam War America’s had not been.

Powell may have been right about the Vietnam War, but not about Clausewitz. Powell had misread the final section of the opening chapter of On war, that which describes war as ‘a strange trinity’. Its three elements are not the people, the army and the government, but passion, chance and reason. Clausewitz went on to associate each of these three elements more particularly with the feelings of the people, the exercise of military command and the political direction of the government. But in doing so he moved from the ‘trinity’ itself to its application. The people, the army and the government are elements of the state, not of war. The distinction is crucial not only to the relevance of On war today but also to what follows in this book. Clausewitz informs much of its content; he does so because he emphatically sought to understand war in terms as broad as possible, and to be as little bounded by the circumstances of his own time as his not inconsiderable intellectual powers enabled.