The problems of terrorism and counterterrorism are closely related to other themes stressed in this volume: sovereignty; state building, nation building, and peacebuilding; norms; responsibility to prevent and protect; legitimacy of the use of force; failed, failing, and fragile states; and democratic transitions. In the second decade of the twenty-first century, the threat of terrorism emanated principally from proponents of radical Islamism, as it did during the first decade. Finding an effective response remained a challenge. Although terrorism receded as a global threat, it contributed to state failure and civil war and continued to provoke military intervention from targeted states.