Armed Guests: Territorial Sovereignty and Foreign Military Basing. By Sebastian Schmidt. New York: Oxford University Press, 2020. 294p. $74.00 cloth.

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 byNGOs. 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 policymakers, especially in the areas of human rights, NGOs, and international law.