Terrorist Attack on the U.S. Homeland

Terrorist Attack on the U.S. Homeland

A mass casualty terrorist attack on the U.S. homeland or a treaty ally


Al-Qaeda and other terrorist organizations continue to pose a stark threat to the U.S. homeland more than a decade after 9/11. Although the United States has been successful in eliminating most of the senior al-Qaeda leadership, affiliates and other extremist groups—the self-described Islamic State, al-Qaeda in the Arabian Peninsula (AQAP), and Lashkar-e-Taiba—have emerged and grown in strength. In December 2014, Secretary of State John Kerry described the Islamic State as “a danger and a threat to the interests and the values of all of us,” and emphasized the “acute threat” of foreign fighters returning to their homes in Western countries. 

Just the year before, the White House stated in August 2013 that AQAP “poses the greatest potential threat” and demonstrates “an interest in and a willingness to attempt serious attacks on the United States, our allies, and our people.” That same month, AQAP threats resulted in the closing of more than two dozen U.S. diplomatic facilities. Domestically, concerns of homegrown terrorism were reinforced by the April 2013 Boston Marathon bombings.

In an effort to prevent mass casualty attacks, the United States has taken steps—as stated by then-deputy national adviser John O. Brennan in 2009—to “hold state sponsors of terrorism accountable for their crimes.” Although the 9/11 attacks were a rare catastrophic event, another attack of the same scale is plausible. A mass casualty attack would demand an immediate response from the United States, threatening to implicate the country in protracted conflict, as was the case with the war in Afghanistan following 9/11.

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