Answering the question as to why some civil wars end in a matter of months, while others persist for decades (Afghanistan, Angola, Burundi, and Colombia), is both theoretically interesting and critical to the design of more effective methods for ending intractable civil wars. David E. Cunningham's Barriers to Peace in Civil War stands as a lively addition to a growing scholarship on this important topic. The author begins by pointing out the huge variation in the duration of wars, with the vast majority ending in a matter of months or years but a significant minority carried on for many years, if not decades. He explains why we should care about these seemingly endless civil wars, as they are responsible for far more casualties, are more likely to involve genocide, and are also more likely to recur.

To address this question, Cunningham first reviews explanations in the literature for the duration of civil wars. For instance, some conflicts may last longer because they are fought over things that cannot be divided, such as when the conflict is waged over control of national territory (Monica D. Toft, The Geography of Ethnic Violence: Identity, Interests, and the Indivisibility of Territory, 2003) or sacred spaces (Stacie E. Goddard, “Uncommon Ground: Indivisible Territory and the Politics of Legitimacy,” International Organization 60 [2006]: 35–68; Ron E. Hassner, “‘To Halve and to Hold’: Conflicts over Sacred Space and the Problem of Indivisibility,” Security Studies 12 [2003]: 1–33). Alternatively, informational asymmetries may cause one or both sides to overestimate their chances of prevailing in battle, inducing them to fight on (Branislav L. Slantchev, “The Power to Hurt: Costly Conflict with Completely Informed States,” American Political Science Review 97 [2003]: 123–33). It may also be that the winning side (usually the government) cannot credibly commit to protecting the losers once they disarm, making the losing side unwilling to lay down their weapons (Barbara F. Walter, “The Critical Barrier to Civil War Settlement,” International Organization 51 [1997]: 335–64; Barbara F. Walter, Committing to Peace: The Successful Settlement of Civil Wars, 2002; James D. Fearon, “Why Do Some Civil Wars Last So Much Longer Than Others?” Journal of Peace Research 41 [2004]: 275–301). Finally, civil wars may involve wartime looting, creating pecuniary disincentives for the combatants to reach a settlement (see Paul Collier, Anke Hoeffler, and Måns Söderbom, “On the Duration of Civil War,” Journal of Peace Research 4 [2004]: 253–73).

Cunningham discusses each of these factors in turn, concluding that “they cannot, however, come close to explaining the extreme variation in civil war” (p. 12). This is because many wars end quickly despite incentives for war profiteering, the apparent indivisibility of the stakes of conflict, and informational asymmetries, while other wars last for decades despite the relative absence of such barriers to peace.

The author then advances an alternative explanation that the number of “veto players” (“a set of actors that have separate preferences over the outcome of the conflict and separate abilities to block an end to the war”; p. 15) largely determines how long a conflict will last. Prolonging the war is, according to the author, something veto players do deliberately “because, in the end, it gets them a better deal” (p. 4). Cunningham argues that the more veto players there are in a given conflict, the more difficult it is to collectively agree on a division of state resources that all participants prefer to the continuation of war. This is the major reason why some conflicts are easier to end than others—because “spoilers,” or veto players, emerge to scuttle the peace agreement (Stephen John Stedman, “Spoiler Problems in Peace Processes,” International Security 22 [1997]: 5–53).

To test these hypotheses, Cunningham conducts statistical analyses on 200 civil wars since World War II, demonstrating that wars that involve more participants last longer than those with fewer. He then conducts a controlled comparative analysis on two civil wars in the Great Lakes region (Rwanda and Burundi), showing that despite their many similarities, the Burundi conflict was much more difficult to resolve because there were more veto players in the conflict. This created incentives to hold out for a better deal, shifting alliances and, therefore, information asymmetries about each player's expected likelihood of prevailing in war. As a result, the negotiations to end the Rwandan conflict were relatively expeditious, whereas negotiations to end the Burundi conflict were very complicated, resulting in another 14 years of war.

Despite its powerful argumentation and trenchant analysis, the book has its weaknesses, a fact that the author himself recognizes. First, there is the problem that the number of veto players is used as a proxy for player preferences—with more veto players indicating more divergent preferences. However, more veto players does not necessarily mean more divergent preferences. A large number of veto players will not upset or hinder a settlement so long as the players' preferences are closer to one another than they are to the status quo; by contrast, a smaller number of veto players can hinder a negotiated settlement if their preferences are such that there is no alternative division that both prefer to the status quo.

A second problem relates to causality. The empirical analysis suggests in certain instances that there are structural conditions that may drive both the number of veto players and the difficulty of peace negotiations. For example, the hard-line Hutu faction (Coalition for the Defense of the Republic, the CDR) that scuttled the peace agreement in Rwanda in 1994 had been a “latent” veto player until events in neighboring Burundi and the actions of the international community convinced the hard-liners that the peace deal would yield an unsatisfactory outcome, leading them to “spoil” it (p. 175). This indicates that, at least in some cases, the number of veto players may at best be an intervening, rather than independent, variable in explaining the success of peace negotiations. If this is true, then the fundamental drivers of stalled peace agreements might lie farther up the causal chain.

Despite these problems, Cunningham has written a highly important and persuasive book on the impact of the number of conflict parties on the likelihood of successful conflict resolution. The policy conclusions are fascinating, if a bit vague: Include no more and no fewer than the actual number of veto players in the negotiation process—too few leads to spoiling by the omitted player and too many increases the difficulty of finding a viable settlement. Identifying all of the relevant veto players in turn “necessitates a deep understanding of the individual players involved in conflict and in society” (p. 254). This is a strong a call as any for involving country and conflict experts in the negotiation setting—for getting the process right may make the difference between an expeditious resolution and one that is many years off.