In January 2009, the head of MI-5, Britain's domestic security service, stated that 2,000 individuals in Britain were directly connected to Islamist terrorist plots and that many more supported terrorism through fundraising or propaganda. From September 11, 2001, to the end of March 2008, British authorities arrested 1,471 individuals on terrorism-related offenses. This report analyzes data on major Islamist terrorist plots in Britain and reveals that individuals who traveled in Pakistan and received terrorist training there or in Afghanistan are a central part of the challenge of Islamist terrorism in Britain. Because al-Qaeda's strategy relies partly on using European nationals to carry out attacks against the United States, the rise of Islamist terrorism in Britain and Europe poses a serious danger to the U.S. and its allies in Europe and around the world.