An impressively large number of politicians, opinion makers, scholars, bureaucrats, and ordinary people hold that terrorism -- and al Qaeda in particular -- poses an existential threat to the United States. This alarming characterization, which was commonly employed by members of the George W. Bush administration, has also been used by some Obama advisers, including the counterterrorism specialist Bruce Riedel. Some officials, such as former U.S. Secretary of Homeland Security Michael Chertoff, have parsed the concept further, declaring the struggle against terrorism to be a "significant existential" one.

Over the last several decades, academics, policymakers, and regulators worldwide have developed risk-assessment techniques to evaluate hazards to human life, such as pesticide use, pollution, and nuclear power plants. In the process, they have reached a substantial consensus about which risks are acceptable and which are unacceptable. When these techniques are applied to terrorism, it becomes clear that terrorism is far

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