In order for a war to be justifiable, or so we learn from most books on the subject, it must be waged by a legitimate authority. For a long time, the most obvious locus of that authority has been the sovereign state, and to the extent that international institutions could ever claim to possess such authority, this was by virtue of prior agreements among the former. But as has been pointed out, this basic equation of legitimate authority with state sovereignty is problematic, since it implies that there cannot be any legitimate authority over and above that of states. In the absence of any such overarching authority, it becomes hard to see how the doctrine of just war could be anything more than an instrument of state interest, by providing a convenient way of demonizing the opponent in times of war. As one critic has remarked, ‘in a world of sovereign states, a formula whose righteousness is in the eyes of the beholder is surely fatally flawed’.

This basic difficulty has been with us at least since Grotius, but has been further aggravated by the recent emergence of new kinds of warfare and the rise of new actors. Many of the wars being fought today cannot easily be classified as either international or domestic in character, simply because most of them are both simultaneously. Many of those wars also involve non-state combatants, such as terrorist networks and private security firms.