At the core of NATO’s reticence in codifying its decade-long contribution to the fight against terrorism in an agreed policy lies a definition challenge. The incidence, nature, scope, and, above all, perception of the threat posed by terrorists vary enormously among countries and regions. To provide a common definition of what constitutes a terrorist is an exercise of drafting acrobatics, impossible even for the most skilled and experienced NATO policymaker. Yet the very nature of NATO—a political-military organization for the collective defense of its members’ territories and populations from external attacks—drives its need to identify where an attack is coming from and who the enemy is. In the case of the fight against terrorism, the Alliance instinctively needs to define who and where the terrorists actually are. Terrorism, like war, is ultimately a means to an end, not an end per se. For many years, in the collective psyche of NATO’s integrated structure, to fight against terrorism without identifying the adversary was like fighting war itself. The lack of a clear opponent denied planners and diplomats a critical element of NATO’s defense paradigm. Consistent with this logic, the 1999 Strategic Concept made only indirect reference to acts of terrorism as one of many security challenges and risks together with sabotage, organized crime, and the disruption of the flow of vital resources. On the other hand, the nature of terrorist acts has long been perceived, especially in Europe, as deriving from “internal” motives—from separatism to political extremism and anarchism.