Clausewitz made the intuitively appealing claim that wars tend to “absoluteness,” and that all limitations imposed by law and morality are in theory alien to it. Clausewitz of course knew that there are in practice many limitations to how wars are fought, but he saw them as contingent to what war is. Since then, however, historians such as John Lynn (Battle: A History of Combat and Culture [Westview Press, 2003]), John Keegan (A History of Warfare ([Random House,1993]) and Victor Davis Hanson (The Western Way of War [Oxford University Press, 1989]) have taught us to see things differently: war is a cultural phenomenon, and the limitations that rituals and taboos impose are essential to what war is. With Conspiring with the Enemy, her intelligent and erudite book on cooperation in war, Yvonne Chiu builds on that work by showing the wide variety of forms cooperation in war can take—something that, Chiu claims, we tend to overlook and take for granted at the same time.

But not only is there cooperation in war; there is even an ethic of cooperation: the belief that cooperation is good, even if it goes against one's own (immediate) interest. Most familiar are the instances of cooperation in war that are aimed at sparing enemy lives, for instance by not shooting at lone soldiers who form too easy a target. This is the “naked soldier” (36) from Robert Graves's memoirs, brought to fame by Michael Walzer's Just and Unjust Wars (Basic Books, 1992). The underlying intentions vary: sometimes it is the desire to save the other person's life; at other times, it is the wish to save one's own soul. Sparing a life is sometimes mainly about the soldier's self-image as something other than a murderer.

This avoiding of “too easy” killing falls under the broader category of cooperation aimed at ensuring a “fair fight,” and Chiu illustrates with many examples how soldiers have felt uncomfortable with sniping and ambushing—as some feel uncomfortable today about drone killing. At the same time, she points out that there is something undeniably odd about the idea that war should provide combatants with equal opportunity; although occasionally compared with sport, war is a very different activity, and Chiu believes that reluctant shooters are “positing a category mistake” (42). The naked soldier either lacks competence or a correct appraisal of the enemy's skillfulness, and war is testing those very things. Military and political leaders like their wars asymmetrical and tend to avoid putting soldiers in the fair fight these soldiers themselves might prefer.

A second broad category of cooperation is aimed at exempting certain categories of people from attack, such as prisoners, clergymen, and medical personnel. Saving civilian lives as such, an essential part of the just war tradition, is less often the aim of cooperating militaries, at least until fairly recently. In the not too distant past it was not uncommon for armies to plunder, rape, and kill the local population. Although such tactics resemble those of some of today's insurgents, we did witness the rise of “a multidimensional system of layered cooperation” to safeguard civilian life (104). The obligatory uniforms for military personnel are a case in point. Wearing uniforms seems irrational as it makes one easily identifiable as a legitimate target, but when all parties in a conflict do it, this greatly reduces the risk to civilians. Interestingly, soldiers are also willing to run considerable risks to prevent civilian casualties when enemy forces fail to take measures to avoid such casualties, or even target civilians deliberately. Even this unilaterally upholding the rules of war is still aimed at cooperation, Chiu thinks, albeit in the future, sending an important signal to the enemy, fellow soldiers, the international audience, and future participants. Such forms of “intelligent restraint” (35) are difficult to recognize as cooperation, however, and Chiu acknowledges that she stretches the definition of cooperation fairly far here.

A third and final category of cooperation is aimed at ending a war quickly; think of the short and decisive battles of hoplite warfare in ancient Greece, where citizen soldiers could return to their farms at day's end because all involved accepted the result of a few hours’ fighting as decisive. Of all forms of cooperation in war, this is the one that reduces harm the most. Regrettably, it has disappeared, and we will not see it back anytime soon, mainly because we have forgotten that we can also cooperate on the format of war, and would no longer accept the clear but somewhat arbitrary outcome of a short battle.

Having described these three broad categories of cooperation, Chiu takes a different tack, describing the limitations of cooperation in war. We now get to see the abovementioned reluctant snipers in a tragic light: their acting honorably to ensure a “fair fight” can lengthen a war, and thus increase the total amount of harm it causes. The more unbearable a war is, the sooner it will be over. Taking good care of prisoners of war, for instance, leaves a warring party with fewer resources with which to fight and win. So does providing equal medical care to wounded enemy soldiers and to one's own. As Chiu states in a later chapter, the immunity medical personnel enjoy (which they would not enjoy had they not been neutral) is now questionable since this care has become so good that it works as a force multiplier by returning injured soldiers to duty and by boosting morale because soldiers know that everything possible will be done to rescue them from the battlefield if wounded.

The first five chapters on cooperation and its limitations form the best (and largest) part of the book. They are followed by two chapters that discuss a wide variety of subjects ranging from the moral equality of combatants to terrorism. Although there is a great deal here that is relevant to current debates in just war theory and military ethics, they lack the focus of the earlier part of the book. Chiu returns here again to the examples of medical immunity and neutrality and the treatment of prisoners, which are interesting and relevant but in themselves not of the greatest consequence. The broader point she wants to make seems to be that there is more “irrationality” in war than meets the eye. In that sense, Chiu, apparently herself amazed by all the counterintuitive forms of holding back in circumstances where all was supposed to be fair, is still under the spell of Clausewitz, who held that “kind-hearted efforts to minimize bloodshed misapprehend the logic of war” (146). Chiu ends with the sobering conclusion that well-intentioned efforts to limit harm might do more harm than good, while we will not see the kind of structural cooperation to end war quickly that would really spare us much of war's harm.