Written in the wake of the invasion and the resulting occupation of Iraq by US-led forces, the preface to the fourth edition of Michael Walzer's Just and Unjust Wars (Basic Books, 2006) reiterated his long-standing view that intervening militarily in a state to overthrow its government is unjustifiable. He did not, however, think that rejecting all-out war in this form necessarily entailed doing nothing when faced with a regime such as Saddam Hussein's. An alternative was to engage in “force short of war.” Actions such as the establishment of no-fly zones could serve limited humanitarian goals without violating the principle of self-determination on the basis of which Walzer more generally upholds the Millian principle of nonintervention. The decade and a half since Walzer's comment has seen an extraordinary growth in practices of force short of war—methods including limited strikes; the deployment of special forces in short, narrowly defined operations; no-fly zones; and drone strikes. It has also seen the growth of a philosophical literature—spearheaded by Daniel Brunstetter and Megan Braun—developing a theory of limited force. Brunstetter's new book is a landmark work in the development of this theory.

The book's title testifies both to Brunstetter's indebtedness to Walzer and his determination to challenge the accounts of just war that Walzer and his followers laid out from the 1970s onward. Brunstetter argues that ethical and legal thought are hidebound by a dualistic understanding of force. With the exception of revisionists and cosmopolitans like Jeff McMahan and Cécile Fabre, most “legal and just war scholars” accept a fairly clear cut “war-peace dichotomy” (16). They think (or they theorize as though they believe) that permissible force falls into two categories: in circumstances demanding large-scale, politically ambitious uses of force, just war theory applies along with UN Law and International Humanitarian Law and, together, they grant fairly wide permissions to states and combatants. In all other cases, the appropriate framework is “law enforcement.”

The need for a theory of “just and unjust uses of limited force” arises partly from the experience of intermediate zones, spaces whose type and intensity of disorder places them somewhere between war and peace proper. These zones suffer from “fragmented” or “contested” sovereignty, often owing to problems such as the dominance of rebel groups in geographic pockets falling within a state's territory. Terrorist threats issuing from these spaces and the need to restore rule of law for those living under their sway mark the failure, by definition, of law enforcement. And yet it is by no means clear that these spaces suffer from or are liable to the prosecution of war. The temptation that governments in thrall to binary thinking face is to extend just war norms to encompass this sort of challenge in their attempts to establish security and peace. While some cases might be amenable to a more sustained, resolute employment of military force, many cases will not. And to apply in bello permissions or to stretch law-enforcement norms to accommodate the actions that might be needed is both ill-suited to the problem and subversive of the norms governing peacetime.

Properly understood, then, the ethics of war only covers some of the cases where force going far beyond law-enforcement norms may be necessary—other cases are amenable to vim that would not have been amenable to large-scale warfare. But, on the other hand, the aim of the new theory is not only to fill in a normative gap; it is also, to some extent, to contest the terrain claimed by just war theory. Brunstetter also sees vis (force) as important owing to the recent failures of war. This theme is most prominent in the chapter on jus post vim (justice after the use of force). Brunstetter argues that, if we are to work out systematically the principles of limited force initiation and conduct, then we should take as a starting point the principles governing the aftermath of limited force. These are constrained by a severely truncated conception of “victory.” Whereas a more ambitious vision of victory worked out rather badly, he maintains, in Afghanistan, Iraq, and Libya, the much more modest goals that limited strikes and operations can aim at, such as restoring a modicum of order in contested territories, are more likely to prove feasible. These more modest goals indicate, he maintains, that the principles governing the initiation and conduct of limited force should be restricted too.

Part 1 of the book offers two chapters surveying recent cases in France, Mali, and elsewhere that pose the most acute problems for a war vs. law enforcement binary, and the debates in war ethics that responded to the problem over recent decades. Brunstetter makes a compelling case here for the need for fresh thinking. The theory of limited force that he offers in reply (making up parts 2 and 3) aims to shore up legal restraints threatened by just war norms when they are applied in the face of threats like those posed by ISIS affiliated terrorists (53). It does so by adapting just war principles for the much more limited purposes of smaller-scale missions with constrained goals. Architecturally, the theory of limited force imitates the structure of just war theory. The ethics of resorting to limited force is set out in a theory of jus ad vim. The just conduct of force is covered by jus in vi. And the endings of missions of limited force are considered under the headings of jus ex vi and jus post vim. These dimensions of just force theory are devised in Brunstetter's careful analysis by adapting their just war counterparts to the special and generally much less ambitious and more constrained goals of limited force. While “just cause” expands to permit a wider range of more modest goals by comparison with its role in jus ad bellum, the theory more generally tightens restrictions. Often, the reason for doing so is to ensure that employments of limited force do not morph into full-scale war. To the usual list of criteria associated with jus ad bellum, therefore, the jus ad vim adds a focus on “probability of escalation.” There is a strong presumption against decisions supporting expansion towards wider engagement and full-scale war. Similarly, in the jus in vi, standard in bello principles are interpreted in more tightly restricted ways, guided by a “predisposition toward maximal constraint.” Brunstetter argues that, unlike in war, in cases of limited force, combatants should not be treated as legitimate targets unless they pose an active threat. By contrast, in wars, combatants are held to be liable to attack at any time within a war zone whether or not they pose an imminent threat to any specific target. Similarly, the expectation of collateral harm to civilians is considerably reduced in cases of vim compared with war through a much more restrictive principle of necessity.

The persuasiveness of Brunstetter's argument from post conflict principles to principles governing the initiation and conduct of force depends on how one interprets the roles of the different parts of just war theory. Jus post bellum is certainly the dimension in which one considers war endings but Brunstetter also closely identifies it with the theory of war ends (i.e., goals, purposes). Some theorists might see the theory of ends as belonging more squarely within the jus ad bellum (particularly as being defined by the theory of “just cause” and “right intention”) while jus post bellum arises from the need to specify residual responsibilities attributable to different parties after the ends of force have been achieved. Brunstetter also has things to say about “punishment of evil” and retributive uses of force that philosophers chary about the idea of punitive war will find controversial. In cases where vim fails to reestablish rule of law, for instance, he suggests that “framing the use of force as punishment” rather than treating it as a means of “defense” will achieve greater constraint (242–52, 257).

Brunstetter makes a strong case for the distinctiveness of limited force compared with law enforcement and war. His presentation of a systematic account is an exciting and valuable contribution to the literature on the ethics of war and violence. Richly illustrated with examples from recent cases, it will be essential reading for anyone working in the wider field of the ethics of armed conflict but especially those who are interested in smaller-scale uses of force by states.