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 and reducing barriers among national law enforcement authorities so that information could be meaningfully shared and suspects apprehended expeditiously. 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, issues that have been receiving renewed attention in light of growing European concerns about the possible threats posed by European fighters returning from the conflicts in Syria and Iraq.