With typical trenchancy, Oliver O'Donovan's The Just War Revisited turns most current approaches to just war on their head. On the one hand, he argues that taking up arms in a just cause is not so much the right of legitimate states as it is their duty. On the other, he shifts the centre of discussion from the usual checklist of restrictions imposed on a belligerent state (e.g., just intention, discrimination and proportionality) to an analysis of just warfare, from its initial consideration, through its implementation, to its conclusion, as the working out of an ongoing act of judgement. By judgement O'Donovan means three things: (a) the determination that there is a case for wrongdoing; (b) the rendering of a judgement in the case; and (c) taking action to remedy the injustice. O'Donovan calls these three moments of judgement description, discrimination and foundation, respectively.

This notion of judgement, which entails both making a determination of right and wrong and taking the necessary action to make things right, sums up O'Donovan's understanding of the legitimate role of government. O'Donovan cites Rom 13:1–7 in this regard: ‘Rulers are not a terror to good conduct, but to bad’. O'Donovan's application of this analysis to the role of governments in international affairs bring us to the central argument of this book. When a state acts against the common good or victimises its own people, who is to exercise judgement on behalf of the world community, which has a stake both in worldwide security and the promotion of justice at the local level? O'Donovan's answer to this question is unequivocal: When international order is threatened by a renegade state, or when human rights abuses call for redress, some means of rendering judgement and satisfying justice is required.

In large part, O'Donovan looks to the United Nations for the exercise of judgement on the international scene. However, as O'Donovan is quick to point out, the United Nations is an organisation, not a government. It possesses no executive authority that is independent of the member nations that constituted it (nor is there any indication that O'Donovan would favour any move in that direction). So the United Nations must act in concert with individual governments or coalitions of governments which (a) possess the military capacity to act effectively against wrongdoers and (b) have a demonstrably immediate and genuine care for the well-being of the people whose cause is being defended (e.g., cultural ties, regional solidarity, shared religion).

O'Donovan is right to claim that he is invoking the classical Christian teaching regarding just war. From Augustine to Grotius, waging war justly has meant defending the weak against injury, whether this means the state acting in defence of its own people (and of the downtrodden among its own people), or acting in defence of other people who have no one looking out for them. At its heart, this teaching assumes that the state is not the embodiment of the people, but its advocate and protector. This sums up the state's brief, and is its sole raison d'être. In a tantalising aside, O'Donovan associates this vision of the state with the emergence of the ideal of Christian knighthood in the twelfth century. He implies that, just as the knight must always be available to the stranger in need, so any government worth its salt stands ready to be on loan to a people in distress.

As O'Donovan is quick to note, this approach to just war makes it harder to justify defensive action and easier to defend intervention on behalf of others. Defensive war can only be justified if it is also undertaken in defence of the principles that justify intervention (e.g., recognition of a common humanity, shared stewardship of the planet, mutual accountability with regard to human rights, respect for the dignity and self-determination of weak nations and sensitivity to local cultures and customs). On the other hand, these criteria constitute a body of law that positively mandates international action against outlaw states and on behalf of aggrieved peoples.

What is the basis of this body of law? Most concretely, it is the tissue of treaties on which the United Nations rests. Behind this lies the ius gentium (law of the nations), which is a loose body of treaties, conventions and appeals to nature that served as a placeholder for international law from the late Middle Ages into the early modern period. Behind all this, for O'Donovan, stands the biblical witness, which asserts that harmony and mutual love, not competition and survival, are the principles that should govern the actions of individuals and states. In the final analysis, this witness must be taken on faith.

This book furthers the conversation about just war in several ways. To begin with, it changes the conversation significantly to shift its focus from the right to wage war to the duty to do so. How differently might things have shaped up if the international community had intervened to stop the genocide in Rwanda? And how might a stronger United Nations mobilise individual nations to act in the event of a similar crisis in the future? Rethinking just war in the direction of policing and rescue clarifies the role of the United Nations and of its member nations in maintaining peace. There are occasions when war must be waged for justice's sake and in the cause of peace. There is no question but that Christian tradition supports this.

Again, by linking just war to the act of judgement, O'Donovan blurs the distinction between warfare and policies which lie this side of war. War is not state policy pursued by other means; rather, it is an extension of coercive strategies which fall short of armed conflict, but still register hostility to renegade states in ways calculated to force capitulation. O'Donovan's discussion of economic sanctions is illuminating. He argues that sanctions are not an alternative to war, but a form of war, subject to discrimination and proportionality, but also dependent for their effectiveness on a clear will to raise the stakes if necessary. In other words, once a judgement has been made that there exists a wrong to be righted, the international community is free to gauge its response. But the decision to impose sanctions (or even simply to apply diplomatic pressure) should not serve as a way to postpone a judgement of wrongdoing that communicates the intention to redress the wrong by all proportionate means.

This book raises one red flag for me. As I mentioned above, O'Donovan labels the three moments of judgement, description, discrimination and foundation, respectively. It is the last term that troubles me. O'Donovan explains that this aspect of judgement has to do with the victor's responsibility for postwar rebuilding. But ‘foundation’ resonates clearly, and a bit too closely, with Hannah Arendt's notion of a violent act that founds a political society. If O'Donovan has Arendt in mind here, he needs to clarify what he means to be saying. He surely does not want to identify the act of war with a prepolitical act of violence. Quite the opposite: in his view, a just war is the act of a mature political community. Yet I remain concerned – perhaps because, in his insistence that his approach to just war is ‘radically evangelical’, O'Donovan exposes himself to the suspicion that the judgement he espouses might not be as accountable to the international community as he claims. After all, large portions of that community do not view the gospel as authoritative. If O'Donovan were willing to draw natural law into the mix, it would go a long way towards giving his powerful insights into just war a wider reach.