What factors explain the recent growth in the enforcement of international law by nongovernmental organizations (NGOs)? Why do some NGOs engage in enforcement activities instead of more traditional NGO activities such as advocacy and service delivery? Mette Eilstrup-Sangiovanni’s and J.C. Sharman’s answers to the questions lie in both the increasing demand for enforcement and in the enhanced ability of NGOs to meet it. First, they argue that, on the demand side, the number and scope of international treaties have exploded in recent decades. However, as is widely known, not all states are compliant with their commitments, thereby increasing the need for enforcement mechanisms. Second, on the supply side, they contend that technological innovations and the increased access of NGOs to the global governance system have greatly enhanced their role as law enforcers. Finally, and perhaps the most novel aspect of the approach, is their view toward competition between NGOs. An increasing number of NGOs operate transnationally; however, these NGOs must compete with one another for scarce resources, including financial backing, technical expertise, and volunteers. Rather than seeing this competition as causing NGOs to change their activities to better fit into the hegemonic narratives created by larger, better-funded organizations, Vigilantes beyond Borders contests that the struggle over scarce resources leads to an incentive to differentiate one’s actions from the others by way of “niche” actions like enforcement.

The book’s conceptualization of transnational enforcement provides three unique contributions to the literature. First, it clarifies the role of NGOs in global governance. Eilstrup-Sangiovanni and Sharman define transnational enforcement as “action aimed at compelling (rather than merely encouraging or facilitating) compliance with the law” (p. 17). More specifically, they develop a typology that ranges along a spectrum from indirect actions (e.g., monitoring, surveillance, evidence gathering, and investigation) to direct actions (e.g., civil or criminal prosecutions, physical intervention to halt illegal actions, and even temporary detention of transgressors). Second, this concept encourages scholars to consider such enforcement of international law by NGOs as a legitimate and independent form of law enforcement, separate from the will of states or intergovernmental organizations. Third, the authors present an intuitive theoretical argument about why NGOs have engaged in such activities at an increasing rate. At the heart of this theory are technological change and the shift toward greater inclusion of NGOs in global governance. This argument challenges the traditional view of the tools at NGOs’ disposal, going beyond their ability to persuade and strategically use information to leverage states to action. Dating back to seminal works by Martha Finnemore and Kathryn Sikkink on the norm lifecycle, or Margaret Keck and Kathryn Sikkink on the “boomerang model” of transnational advocacy, scholars have conceptualized NGOs as lacking in the ability to directly coerce violators of international law. Vigilantes beyond Borders challenges this notion by incorporating evidence from the direct enforcement of international law.

Chapters 2–4 provide rich illustrations of the different mechanisms that NGOs take to enforce international law in these areas, including, but not limited to, investigations, evidence gathering, civil and criminal prosecutions, and physical interventions. The presentation of these cases is masterful and ambitious, tackling three domains: human rights (chap. 2), environmental protection (chap. 3), and anticorruption (chap. 4). Human rights are more solidified into international law than the other two domains and, as mentioned, safeguarding them has been the dominant issue domain among advocacy scholars. Similarly, environmental protection has seen a massive growth in the number and scope of treaties since the rise of the modern environmental movement in the 1970s. Anticorruption efforts are comparatively newer on the international stage, presenting an interesting case study by which to illustrate the authors’ theoretical predictions in an emerging arena.

Each issue-area chapter is organized around the authors’ central premise of the demand for enforcement and the supply of this enforcement by NGOs. On the demand side, the authors address how the enforcement gap has grown in recent years. These sections highlight major additions to international law via treaties and the creation or expansion of intergovernmental organizations. Next, and turning to the supply side of their argument, these chapters outline the legal drivers behind NGO enforcement. The expansion of access for NGOs has been accomplished via the actions of domestic or international courts. In the area of human rights, for example, allowing prosecutions by private actors was instrumental in holding perpetrators accountable for crimes committed during the periods of military junta rule throughout Latin America. Then, the authors present the ways in which technological advancements have enhanced the work of NGOs. These include social media, phone cameras, and satellite imagery, which have been crucial in investigations and evidence gathering. Finally, these chapters end with a discussion of how competition among like-minded NGOs has incentivized the differentiation of roles and the advent of what the authors refer to as “vigilante enforcement.”

A question that requires further fleshing is at what level the enforcement gap and competition among NGOs incentivize more enforcement work by NGOs. The theory behind this process is intuitive: treaty commitments outpace the ability of states to enforce compliance, and as more NGOs develop in an issue area, an organization sees a greater need to differentiate itself from the pack. Yet, this differentiation process is taking shape in all three domains, despite human rights being significantly more embedded in the international system than the newer area of anticorruption. Exploring a counterfactual case would help illustrate the explanatory power of the theory and provide an avenue for gauging this benchmark when operationalizing these concepts in further empirical evaluations.

A more minor concern is the need to provide a more solid conceptual framework to distinguish direct from indirect enforcement. As the authors state, it is difficult to conceptually separate these processes because this is very much a gray area. However, to test some of the authors’ hypothesized relationships, it is important to clarify this division conceptually. Admittedly, this task is difficult, given that it is impossible to ever know the exact motivations underlying individual actions taken by NGOs, yet future work should tackle it.

These critiques should not be taken as diminishing the core arguments of the book but rather as pointing out avenues to address in future research. Indeed, the book’s greatest strength is that it provides a major shift in how we think about the work of NGOs and the enforcement of international law. International relations scholars have always conceptualized global governance as pluralistic, and although enforcement is a key component of governance, isolating this role to states ignores this pluralism. Moreover, it would be interesting to see how network analysis research about transnational advocacy networks could be used to understand the decision of these NGOs to pursue enforcement work as opposed to the more traditional roles of advocacy or service delivery. In short, is it the competition between like-minded groups that creates the incentive for differentiation, as the authors propose? Or could it be that the formation of larger networks of NGOs drives the specialization of roles among cooperative actors?

Vigilantes beyond Borders offers a significant contribution to the field and is recommended for scholars, practitioners, and policy makers, especially in the areas of human rights, NGOs, and international law.