9/11, the terrorist attacks on Sept. 11, 2001, in the United States, and the associated events and impact of those attacks.

The attacks, which were carried out by agents of Al Qaeda (a militant Islamic terrorist group led by Osama bin Laden) used three hijacked commercial jet aircraft to destroy the World Trade Center in New York City and severely damage the Pentagon in Arlington, Va. A fourth hijacked plane crashed in Shanksville, Pa., when its passengers attempted to seize the plane from the hijackers. Some 3,000 persons died or were missing as a result of the most devastating terrorist episode in U.S. history.

9/11 was a turning point in the presidency of George W. Bush and U.S. foreign policy, leading directly to U.S. support for the overthrow of the Taliban regime in Afghanistan, where Al Qaeda was based. The attacks were also used to justify in part the U.S.-led invasion of Iraq (see also Persian Gulf Wars) despite the lack of any clear evidence linking the Iraqi government to Al Qaeda, but the impact of 9/11 contributed to strong American public support for the invasion. The Bush administration, which had already insisted on strong presidential powers, asserted that the United States was at war (a response not echoed by the Spanish and British government in the wake of subsequent significant terror attacks in Madrid and London) and that legal restrictions did not exist on the president's powers to defend the country, a position subsequently questioned in part by the Supreme Court.

As a result of the attacks and of the subsequent reports issued by a joint Congressional investigation and by the 9/11 Commission (see below), a number of significant changes to the federal government were made, including the establishment of the Dept. of Homeland Security, which consolidated 22 nonmilitary government security agencies and assumed responsiblity for U.S. air travel security through its Transportation Security Administration, and the establishment of the cabinet-level post of director of national intelligence, who became responsible for overseeing and coordinating all U.S. intelligence agencies. Other far-reaching effects include the passage of the USA PATRIOT Act in 2001 and building-code changes proposed by the National Institute of Standards and Technology in 2005.

The 9/11 Commission, officially known as the National Commission on Terrorist Attacks upon the United States, was established by law in 2002 to prepare a full account of the attacks and make recommendations on how to guard against future attacks. Headed by Thomas H. Kean, the former Republican governor of New Jersey, and consisting of a panel of a five Democrats and five Republicans, it first convened in 2003, interviewed more than 1,000 persons in 10 countries, and issued its report the following year. The commission faced resistance from the White House and the House Intelligence Committee over access to documents and individuals (including the president and vice president), but access to those improved mainly through public pressure brought by the families of the victims of the attacks; the group was not permitted, however, to question directly the detainees at Guantánamo.

The commission held both public and private hearings and issued a report with both public and classified sections. With the benefit of insights dependent on hindsight, it detailed the terror plot's origins, which dated to 1996, and its development, and also identified failures of various U.S. agencies that might have alerted officials to the impending attack or could have led to actions that might have prevented it. Its work revealed problems with U.S. intelligence gathering and interpretation and with law enforcement concerning terrorist threats against the United States, especially with regard to the work of the Federal Bureau of Investigation and to cooperation between the FBI and the Central Intelligence Agency. (It also found no evidence of collaboration between Al Qaeda and the Iraqi government.) Many of its recommendations, which focused on preventing another similar attack against the United States, were subsequently adopted, but thoughtful critics have pointed out that its proposals were limited both by its focus on the hijackings and by an emphasis on centralization of responsibility and control as a solution to overcoming the failures of 9/11.

See the 9/11 Commission's report (2004), the commission staff reports and other materials, ed. by S. Strasser (2004), and the account of the commission's work by T. H. Kean and L. H. Hamilton (2006); P. Shenon, The Commission: The Uncensored History of the 9/11 Commission (2008); studies of the events of 9/11 by L. Wright (2006) and J. Farmer (2009); C. B. Strozier, Until the Fires Stopped Burning: 9/11 and New York City in the Words and Experiences of Survivors and Witnesses (2011); J. Margulies, What Changed When Everything Changed (2013).

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