The predominant vision of war, in the system of sovereign nation-states that evolved in the aftermath of the Treaty of Westphalia, has been one of violence used by nation-states, or alliances of nation-states, against one another. Indeed, as Martin van Creveld in The Rise and Decline of the State suggests, one of the principal functions of the modern state was to wage war. War, in von Clausewitz’s famous formulation, was an instrument of international statecraft that entailed the ‘continuation of politics by other means’. The great wars of the twentieth century to a large extent took this form. It might even be said that inter-state war came to be seen by some as a ‘natural’, if somewhat episodic and terrifying, feature of the modern nation-state system.