The September 11, 2001, terrorist attacks on the United States and the subsequent revelation of Al Qaeda cells in Europe gave new momentum to European Union (EU) initiatives to combat terrorism and improve police, judicial, and intelligence cooperation among its member states. Other deadly incidents in Europe, such as the Madrid and London bombings in 2004 and 2005, respectively, injected further urgency into strengthening EU counterterrorism capabilities. Among other steps, the EU has established a common definition of terrorism and a common list of terrorist groups, an EU arrest warrant, enhanced tools to stem terrorist financing, and new measures to strengthen external EU border controls and improve transport security. Over the years, the EU has also encouraged member states to devote resources to countering radicalization and terrorist recruitment; such efforts have received renewed attention in light of concerns about the threats posed by European fighters returning from the conflicts in Syria and Iraq, highlighted most recently by the November 13, 2015, attacks in Paris, France.