Some arguments are so elegantly and compellingly simple that, as one watches them unfold, one finds oneself wondering why one never saw the world that way before. In Conspiring with the Enemy: The Ethic of Cooperation in Warfare, Yvonne Chiu makes just such an argument. Chiu reimagines war not as the failure of cooperation but instead as an intentional practice thereof. This practice of war has generated over time an “ethic of cooperation” among belligerents that is far more than mere reciprocity or the expression of self-interest (because “it often cuts against that” [p. 24]). Instead, it is motivated by a broader variety of moral and psychological underpinnings, including the sense of fairness. Cooperation in war—sometimes unwittingly—accords one’s opponent “a modicum of respect, trust, and recognition of humanity” (p. 26). Chiu’s startling conclusion is that enemy states not only cooperate in war but also believe that it is right to do so.

Chiu uses a genealogical approach that integrates empirical and philosophical approaches in a very fresh and thought-provoking way. Thus, her book has two parallel aims. Empirically, it aims to unearth some key insights about moral psychology and military sociology, drawing on military and legal history. Philosophically, it aims to establish what fairness, responsibility, and legitimacy mean with respect to war. As a final step, Chiu merges these two approaches to explore how tensions and problems with these concepts might be resolved theoretically and practically. Notably, Chiu draws not only on international law and Western just war thinking (whether philosophical or theological) but also on the historical and philosophical approaches to ethical war fighting of ancient India, China, Japan, and the Iroquois confederation. This approach produces a delightfully readable book rich in concrete historical examples and equally careful and nuanced in its theoretical analysis.

Chiu argues that the ethic of cooperation between antagonists in war serves three purposes, reflecting a broad underlying sense of a common good. First, the ethic of cooperation works to ensure a fair fight, invoking a mixture of professionalism, philosophical principles, and virtue ethics. Fairness, as Chiu points out, has two dimensions, both of which are evidenced here. On the one hand, fairness can mean that the same rules apply to all. As an example, the rule that soldiers must wear uniforms makes “all soldiers equally identifiable and therefore equally vulnerable” (p. 43). On the other hand, fairness can also imply that one should not take undue advantage of one’s opponent. In other words, it is not sufficient to follow the technical rules of competition; one must also stay true to the spirit of competition. As an example, Chiu points to cases where actors chose to restrain themselves from pursuing total victory against opponents who could not offer any reasonable resistance, forgoing massive attacks on troops whose positions were completely exposed. Merging these two understandings of fairness makes it obvious, in Chiu’s telling, why certain types of deception, such as perfidy, are considered inappropriate by a wide variety of societies across time.

Second, the ethic of cooperation minimizes the harm that war causes to certain types of individuals, including civilians, the wounded, and political figures. Here, Chiu’s argument takes a particularly interesting turn as she points out that this cooperation is driven not only by high-minded concerns with humanitarian principles. Instead, Chiu reminds us that protecting political figures from assassination attempts in war passes the costs of war down to soldiers and civilians, in ways that might not be justifiable in terms of the moral and political responsibility borne by the politicians—but that are “undoubtedly beneficial for geopolitical stability” (p. 126). Cooperating to avoid political assassinations thus reflects not only a self-interested reciprocal concern with protecting one’s own leaders but also a set of jointly held interests, including creating trust, reducing conflict, and promoting international order.

Third, Chiu discusses how the ethic of cooperation could help parties end wars quickly and definitively, minimizing the joint damage caused by war. Although Chiu finds evidence that the first two elements of the ethic of cooperation—fighting fair and minimizing harm to protected individuals—remain alive and well, she argues that this third aspect of cooperation in war has disappeared in the modern era. Even though states still aim for short and decisive wars, their focus has turned inward, and “once the cooperative element slips away, the humanitarian potential does too—and the rationale for ending war quickly becomes simply to minimize damage to one’s own side” (p. 150). Chiu finds it unlikely that this third realm of cooperation in war will be revived, largely because our modern understanding of war as a contest between societies is too different from the ancient world’s vision of war as a duel writ large. Furthermore, ancient societies that did have an ethic of cooperation aimed at ending wars quickly and decisively were relatively homogeneous, sharing many values—a condition not mirrored by the current state of international politics.

On this note, Chiu turns to limits on the ethics of cooperation in warfare. One key problem is the potential for tension between different aspects of cooperation. For example, the impetus to cooperate for the sake of a fair fight and the desire to protect particular classes of individuals can clash, as when tactics that could limit the harm to noncombatants—such as the use of snipers or targeted drone strikes (as opposed to ground troops)—are judged to be forms of unfair fighting. Additionally, although in Chiu’s view contemporary warfare norms draw on an amalgam of the ethic of cooperation, just war thinking, and international law, she points out that these diverse normative sources can also conflict. For example, she argues that, whereas the ethic of cooperation itself offers no loopholes to the principle that civilians ought to be spared in war, traditional just war theory’s principle of double effect could justify attacks that knowingly (“if not technically intentionally”) kill civilians (p. 196).

Furthermore, although the ethic of cooperation may arise somewhat organically between modern nation-states, whose war making is “premised on cooperation, not merely in their use of warfare as a political tool, but also in their mutual recognition of one another as entities who can legitimately wage war and of their soldiers as representatives who can legitimately kill on their governments’ behalf,” nonstate actors ranging from independence movements to terrorist organizations are almost definitionally not recognized by their state opponents (p. 196).

And, as Chiu points out, where there is no mutual recognition, war tends to become even more brutally violent. This, then, is the somewhat pessimistic note on which she ends. The “ethic of cooperation alone, even when it does work, is often not enough,” and efforts to further bind warring parties by “aiming to do more”—by incorporating human rights approaches into the norms of war, for example— may actually make things even worse (p. 246). Instead, the lesson to be learned from Chiu’s deep investigation of the history of cooperation in war may be that we need to focus on the “possibly achievable,” rather than aiming for ideal rules whose inevitable violation may lead toward even greater cynicism and loss of trust (p. 249).