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In January 2009, the head of Britain's Security Service (also known as MI5) boasted that his agents were succeeding in cracking down on potentially violent homegrown Islamists. Although conceding that "the battle [was] not won," Jonathan Evans told the Daily Telegraph that his agents were forcing would-be terrorists "to keep their heads down." He went on to note that there were undoubtedly terrorists planning attacks somewhere -- but probably not in Britain.

His optimism, however hedged, was understandable. His interview took place 3 1/2 years after the London terrorist attacks of July 7, 2005. During that period, the British authorities put dozens of would-be terrorists on trial and thwarted numerous attacks. In the immediate wake of 7/7, the Security Service's public...