Patrick A. Mello addresses a set of important questions: Why and under what conditions do democracies participate in armed conflict? Do constitutional provisions that define limits to participation in military actions, the rights of parliaments to veto participation, or public opinion provide effective constraints on democratic leaders? Are conservative, rightist governments more war prone than leftist governments? Or is military capacity—that is, governments’ ability to actually conduct such interventions—the most effective constraint?

One of the most important findings of Democratic Participation in Armed Conflict is that constitutional provisions matter. They provide effective barriers against the participation of democracies, especially for interventions whose international legal basis is controversial. Thus, where international law fails to prevent such wars, domestic constitutions step in—at least in the case of established democracies. None of the democracies with constitutional constraints have participated in such interventions. Another important finding is that military power matters, too, but in ways not anticipated by some theories of international relations. Parliamentary veto points and lack of public support seem to constrain only militarily weak states. Military power alone is not a sufficient condition for participation, but it only works together with other factors, such as the absence of constitutional constraints, public support, or the existence of a right-wing executive. These surprisingly unambiguous findings result from three systematic case studies involving between 23 and 30 democracies for each case.

Mello’s studies start from the observation that the involvement of democracies shows considerable variation, not only regarding military participation or military nonparticipation but also regarding the extent of participation. Some states contribute troops; others provide only logistical support. What is the explanation for their participation in different degrees? And what is the explanation for nonparticipation?

The first chapter provides the scholarly context for the studies, which is set by the democratic peace literature. In an excellently written literature review, Mello carves out the research gap that he seeks to address. The democratic peace literature pays too little attention to gradations of democracies and to different types of domestic constraints. Yet democratic participation varies widely over different cases of military involvement.

The second chapter provides the state of the art on domestic constraints on military involvement by democracies. Here, Mello casts the net widely and discusses three groups of factors in depth, comprising eight separate explanations analyzing the military involvement of democracies. These follow the logic of institutional, constructivist, and realist explanations for democratic participation.

The third chapter presents, as Mello claims, a theory of military participation by democracies that combines domestic factors—including different forms of institutional constraints—and structural factors. However, to speak of a theory of military involvement is certainly an overstatement if one considers the conventional meaning of theory as a set of connected statements providing a causal explanation for a phenomenon. Altogether, Mello develops 10 hypotheses on military (non)participation, establishing a set of necessary and sufficient conditions for each outcome. Apart from the fact that he does not connect these factors to a narrative on how they actually work together to cause an outcome, some readers not familiar with fuzzy-set qualitative comparative analysis (fsQCA) language might find it hard to follow the various types of necessity and sufficiency conditions here, such as INUS and INESS.

Chapter 4 explains the methodological approach of the book—fsQCA. The discussion demonstrates that fsQCA has seen an almost revolutionary development over the last decade in terms of its technical requirements and methodological depth of the discussion. The discussion itself is technical, but necessary for understanding the following empirical chapters. Chapters 5–7 explore democratic war (non)participation in three case studies that occurred within a brief period of time: the Kosovo War in 1999, the Afghanistan War of 2001, and the Iraq War of 2003. The cases are similar, according to Mello, to the extent that they all constitute military actions—actually enforcement measures—that did not have explicit authorization by the United Nations Security Council (UNSC). At the same time, the cases vary, according to the author, as they involve different coalitions of states. In the case of Kosovo, NATO conducted the intervention in its capacity as a regional organization, without proper UNSC authorization. Such an authorization would have been necessary given that NATO acted without the consent of the affected state and as a regional organization out of its geographical area. In the second case of Afghanistan, the intervention was conducted by an ad hoc coalition of willing states expressing their solidarity with the U.S. government after the attacks of 9/11 and acting under the UN Charter’s self-defense provisions. Here, the intervention can be regarded as legal, even if there was no explicit UNSC authorization. In the third case, Iraq, the United States, and the British government led an intervention that did not have UNSC approval. The international legal community was almost united in evaluating the U.S. and British justifications for intervention as not covered by international law.

Each of these case studies is similarly structured: A brief overview and lucidly written narrative of the historical context of the interventions is followed by discussion of the major case-specific explanations for each case. Mello then explains the coding procedure for his key variables: parliamentary veto rights, constitutional restrictions, partisanship, public support, and military power. This is followed by the computer-aided analysis leading to distinct causal pathways for participation and nonparticipation in each armed conflict. The contribution of the findings to the literature—whether they provide support for a hypothesis or not—is then discussed. Finally, Chapter 8 provides a cross-case analysis of the case studies and the book’s key findings.

Democratic Participation in Armed Conflict is clearly structured, and the reader is well guided through the chapters. The author does a very good job of spelling out the contribution to the existing literature. Moreover, Mello makes a case for the superiority of fsQCA over both quantitative approaches and case studies. He stakes out his claims well, but his arguments are not always convincing. For example, it does not become clear why fsQCA should be superior to a quantitative test of the hypotheses apart from the fact that the data are missing with regard to the latter. A clear focus on and argument for fsQCA’s ability to provide a different causal pathway might have been more convincing.

In addition, Mello’s desire to make a methodological contribution besides an empirical one leads to an overemphasis on methods. There is no explanation as to why it is necessary to explain the coding procedure in such great detail for each case study, especially if the procedure is almost identical over the case studies. Here, it would have been more efficient to either explain the coding procedure for all cases in the methods chapter or move these parts to the appendix.

A substantial interpretation and contextualization of Mello’s findings would also have been useful. The most important and robust finding that constitutional constraints matter for war participation is not sufficiently discussed in view of its political relevance. Thus, the most compelling message almost gets lost. Additionally, the relevance of other cogent factors, such as military power, is hard to generalize across the cases, especially as the empirical chapters do not always discuss the same factors. Overall, Mello’s contribution is highly elucidating and provides an important piece of the puzzle to the democratic peace literature, even if it is overtly wedded to and ultimately constrained by its method.