The 11 September 2001 attacks in the US changed the course of world history and made Al-Qaeda a state-like actor in international affairs, thereby confounding a core Realist idea. The event also increased interest in terrorism studies, creating two competing schools of thought within it, the classical and the critical school. The debates between these two broad perspectives have led to many fruitful advances and insights concerning the motivations, methods, and impact of both terrorism and counter-terrorism.