Since the 1990s, international and regional organizations have engaged in a record 17 peace enforcement operations, multilateral military interventions to end violent conflict within a country. In her excellent book, Katharina P. Coleman seeks to explain why states choose to act either through the auspices or with the explicit consent of an international organization. Using a comparative case method, she explores state motivations in the Nigerian intervention in Liberia under the Economic Community of West African States; the U.S.-led operation in Kosovo under NATO; the interventions of Australia in East Timor under the United Nations; and South Africa's intervention in Lesotho and Zimbabwe's actions in the Democratic Republic of the Congo, both under the SADC (Southern African Development Community).

Coleman explains why leading states seek international organization approval by developing a theory of international legitimacy. International organizations are the gatekeepers of legitimacy; they affirm the social rules of state behavior. Drawing on wide-ranging institutional and legal literature, she argues that legitimacy is a “social status judged according to commonly accepted rules” and that “states value international legitimacy as a good in itself” (p. 70). Why? States value legitimacy “because they fundamentally care about their standing in the international society structured by these shared understandings” (p. 41).

The author posits that the rules for international legitimacy judgments in peace enforcement include respect for state sovereignty, prohibition of aggressive war, and recognition that military intervention may be necessary for international peace and security. Her basic thesis is that an intervention is legitimate if it serves the international system; international organizations play a privileged role in deciding whether the interests of the community are served and whether the rules are followed.

Coleman pits the explanation of international organization's conferral of legitimacy against three alternatives. One alternative is rooted in international law. Unilateral intervention is illegal, except in cases of individual or collective self-defense or for maintaining international peace and security. A second alternative is found in neoliberal institutionalism: States utilize international organization approval to share the burden of an intervention and enforce credible commitments. Finally, states choose to obtain international organization approval because these organizations are merely instruments of state power. Thus, hegemons can attain organizational approval at no cost and with ease, the realist explanation.

All five cases provide empirical evidence in support of Coleman's theoretical argument, that states turn to international organizations to make their peace enforcement behaviors acceptable to others. The alternative explanations are rejected. In four of the cases, while lead states often tried to construct a legal argument in favor of enforcement action, in actuality the state did not usually follow legal procedures. In all five of the cases, turning to international organizations did not enable the leader to share the economic burden of the intervention. Leading states paid for the intervention in terms of troops and material. In no case was the international organization merely “cheap figleaves for powerful states” (p. 283), contrary to realist expectations.

But which organization the lead state turns to does matter. There are “rungs in a global legitimacy ladder” (p. 58). The “more an international organisation's membership approaches universality, the greater the legitimising effect of its membership will be” (p. 282). As Coleman concludes, “the UN is better placed than any other international body to signal the world's endorsement of an intervention and thus establish it as a collective operation for the common good rather than a unilateral initiative for national ends” (p. 282).

In her conclusion, the author presents two abbreviated case studies—of Iraq and Lebanon—that underscore the timeliness of her topic. She discusses the importance of international legitimacy with respect to the 2003 U.S. invasion of Iraq. The United States tried to attain UN authorization by the right of self-defense and by Iraq's failure to comply with Security Council resolutions on disarmament. When that strategy failed and the invasion proceeded anyway, the United States sought a UN role in political and economic reconstruction. What is discussed in this case (and not in the others) are the costs of failing to win UN authorization—the weak ad hoc coalition and the decline in U.S. international standing. Thus, Coleman engages in a heated public debate on the UN's relevance. The UN's strength is confirmed, she argues, because the demands of the hegemonic power were resisted and the United States has suffered the consequences of the illegitimate intervention. International organizations are not just instruments of the great powers; as realists assert, “the constraint of international legitimacy cannot be eliminated by an exercise of power” (p. 306).

Coleman has done a masterful job of articulating a nuanced theory of international legitimacy, expanding on the tradition of Inis Claude, Thomas Franck, and Ian Hurd. On a critical issue, she traces rules necessary for legitimacy, and examines the competing audiences to whom legitimacy may be salient—the international, regional, and domestic audiences. Though a useful way to think about legitimacy, more explanation is needed. Among the relevant audiences, the critical one for the theory is the international community. Yet unfortunately, that concept begs for greater clarity and extended discussion.

It is always difficult to analyze cases using a consistent framework. The studies focus on the underlying motivations of the leading state, not on the details of what happened in the enforcement action. These motivations are derived from extensive interviews with the major players in Australia, Belgium, Botswana, Ethiopia, Nigeria, and Zimbabwe. And the author has compiled useful data on state material contributions to peace enforcement operations not found in most sources. At times, the same organization across the five cases becomes repetitive and cumbersome, as the alternative explanations are consistently rejected and the author's theory triumphs.

This book is an outstanding contribution to the literatures on international institutions, peace enforcement across a variety of international arrangements, and international legitimacy.