Rarely can the arguments of a book be so succinctly stated: “In short, peacekeeping intervenes in the most difficult cases, dramatically increases the chances that peace will last, and does so by altering the incentives of the peacekept, by alleviating their fear and mistrust of each other, by preventing and controlling accidents and misbehavior by hard-line factions, and by encouraging political inclusion” (p. 178). Virginia Page Fortna's study has done so. It is brilliantly organized, persuasively demonstrated, and cogently presented.

Fortna sets out to answer three questions: 1) Where do peacekeeping missions go—to easy or difficult places to keep peace? 2) Do they have a significant impact on the duration of peace? 3) How do such missions influence the duration of peace?

Noting that peacekeeping missions were only rarely used before the end of the Cold War, Fortna first estimates a model of duration for civil war cease-fires or peace agreements based on 61 cases drawn from before 1990. The model includes such factors as the levels of infant mortality and democracy prior to the agreement, whether the conflict involved an identity or secession war, the number of deaths and displacements in the war, whether it ended in clear victory or an agreement between the government and opponents, whether the conflict included multiple factions, and whether rebels gained support either from neighboring countries or from control of contraband resources (e.g. timber, diamonds).

With this basic model in hand, Fortna can then answer her first two questions: Are peacekeeping missions sent to keep the peace where the duration of peace is expected to be longer or shorter, and how large is the impact of those missions on the difference between the observed and expected duration of peace? The results are surprisingly strong and robust to numerous specifications. Fortna demonstrates that peacekeepers are sent far more often to deal with difficult conflicts than with easy ones. Other factors also govern the call for peacekeeping: On the supply side, former colonies of, or countries bordering, the United Nations Security Council's five permanent member countries rarely receive peacekeepers. These countries appear to prefer dealing with problems in bordering nations or their former colonies by themselves. On the demand side, requests for international peacekeeping comes most often when rebel groups are fairly strong, so that the government needs help in holding them to an agreement, and demand almost never arises when the rebels or government have been soundly defeated.

The results for the impact of peacekeeping are also strong. For both consent-based missions (UN Chapter VI) and enforcement-based missions (UN Chapter VII), Fortna estimates that, other things being equal, the missions reduce by over half the chance of war resuming. And that is just while peacekeepers are present; if she includes the peaceful years after they depart in the equation, the estimated reduction in the risk of war resuming is 75%–85%! Again, these results are robust across a variety of specifications.

While these findings are enormously valuable by themselves, Fortna goes further, using interviews with the government and rebels (the “peacekept”) and UN officials in Bangladesh, Mozambique, and Sierra Leone to flesh out how peacekeeping missions shift incentives to help preserve agreements. She points out that missions are most effective when they combine monitoring and enforcement functions with real economic rewards to rebels—such as ensuring that rebel leaders obtain government positions and that rebel soldiers are compensated as part of demobilization programs—and thus that missions suffer when rebels have access to their own resources through contraband activities. She also notes that in most cases, where both rebels and governments want peace and mainly lack a means of trustworthy communication and monitoring, even small and modestly armed forces can succeed. However, in cases where the government or rebels have not sought peacekeeping, but peacekeepers are sent to make and enforce peace on them, it is critical to have forces large enough and strong enough to seize resources, hold territory, and win battles against government or rebel troops; otherwise the peacekeepers themselves may become a target and casualty of the fighting.

Fortna makes clear the large variety of useful tasks that peacekeeping missions perform: They are a channel of trustworthy communications, provide impartial monitoring of compliance with agreements, help ensure that postconflict governments do not renege on agreements or target rebels, provide status to rebel leaders simply by recognizing them in mediation, and offer a recourse to either side for reporting and acting on suspected violations or accidental defections from agreements. And that is only for the simplest monitoring missions. More extensive missions may also take control of areas of contraband, denying resources to the rebels that enable them to resist peace; supervise demobilization and disarmament operations; provide training and temporary leadership to new police and military organizations; provide national security in the vacuum often left during negotiation and initial enforcement of peace agreements; and provide funding, guidance, and training to create and staff new government institutions. Such more extensive missions are strongly indicated in countries with substantially failed states, which require not only an accommodation of rebels and reform of institutions but also the creation of a state with legitimacy and authority almost from scratch (as in Sierra Leone).

Fortna's cases are carefully chosen to include those where peace was kept without an external peacekeeping mission (Bangladesh, in the Chittagong Hill region), cases where peacekeeping failed (Sierra Leone in 1999), and cases where peacekeeping succeeded (Mozambique, Sierra Leone in 2001). The author shows how in Bangladesh, a relatively strong government facing a localized rebellion found it preferable to co-opt the rebel leaders with modest concessions to autonomy rather than to invite international peacekeepers who might have insisted on more generous terms for indigenous groups. Yet this also had the drawback that hard-line rebels were not satisfied but remain active and a threat to peace.

In Sierra Leone in 1999, peacekeeping failed because inadequate peacekeeping forces were inserted into a hostile and chaotic situation; this changed two years later when British forces made clear that they would militarily support the UN peacekeeping forces, and aided them in taking control of the rebels' sources of diamonds. Mozambique was the most successful case because there were two well-organized groups (Frelimo in power, and Renamo rebels) who had fought to near exhaustion without a clear conclusion, and both groups wanted an agreement that would bring peace. The main problem was that the accumulated distrust from years of war had created a security dilemma, in which neither group trusted the other side to disarm if it did so, and neither group trusted the other to keep to any peace or power-sharing agreements once made. Because UN peacekeepers were trusted as impartial by both sides, Frelimo and Renamo could deal with each other through the UN, having the peacekeepers channel communication, monitor agreements, implement disarmament, and respond to alleged violations without recourse to resuming war.

This book is an outstanding illustration of how research should be carried out: careful conception of the research problem, scrupulous data analysis, and subtle examination of case studies to better understand and delineate the causal foundations of the results. The importance of those results is evident. As Fortna says, “the answer to the question of whether peacekeeping works is a clear and resounding yes” (p. 173). Scholars and policymakers should pay close attention to these findings, and to her more detailed discussions of how the various capacities of peacekeeping missions can best be tailored to the conditions of specific conflicts. For these findings represent not simply a major contribution to political science but an invaluable contribution to peacekeeping efforts in a world rife with violence.