"Over the course of the last two decades, states have repeatedly turned to local, substate, or nonstate armed groups as partners in counterterrorism or counterinsurgency efforts across the globe. Yet there are significant risks to working with irregular forces-from the risk of enabling predatory militias, human rights abuses and war crimes, to contributing to fragmentation and political instability, to the risk of seeding local conflict or future security threats. Could those consequences be prevented or mitigated? If so, would states be more comfortable engaging with irregular armed groups? This book explores these questions by examining nine U.S. partnerships across Afghanistan, Iraq, and Syria from 2006 to the present. It draws from hundreds of interviews with a range of policy actors and detailed interrogation of crucial policy decision-making moments surrounding U.S. intervention. This wealth of empirical data is used to test and further nuance two important frameworks within international relations theory in ways that substantially advance understanding of transnational and substate security policymaking. Ultimately, the book is about much more than regulating irregular actors. It offers a timely meditation on U.S. security policy since 2001, and on the challenges of transnational policymaking and intervention in fragmented security landscapes"--