As a noun phrase, the term refers to the coordinated terrorist attacks carried out by the Islamic extremist group al-Qaeda against the United States on September 11, 2001. "11 sep" functions as a specific adjectival modifier for the noun "incident." The attacks involved the hijacking of four commercial airliners, which were then used as missiles to strike prominent American landmarks, resulting in the deaths of 2,977 people and causing the collapse of the World Trade Center towers in New York City and significant damage to the Pentagon in Arlington, Virginia.
The operational details involved 19 terrorists who hijacked the aircraft after takeoff. American Airlines Flight 11 and United Airlines Flight 175 were flown into the North and South Towers of the World Trade Center, respectively. American Airlines Flight 77 struck the western facade of the Pentagon. The fourth plane, United Airlines Flight 93, crashed into a field in Shanksville, Pennsylvania, following a passenger and crew revolt against the hijackers, thwarting its intended target in Washington, D.C. The immediate response included the grounding of all civilian air traffic in the U.S. and the initiation of extensive search-and-rescue operations.
This event fundamentally reshaped United States foreign policy, leading directly to the initiation of the "War on Terror," the 2001 invasion of Afghanistan to dismantle al-Qaeda, and the 2003 invasion of Iraq. Domestically, it prompted a major restructuring of national security and intelligence agencies, including the creation of the Department of Homeland Security and the Transportation Security Administration (TSA). The attacks also led to the passage of controversial legislation, such as the USA PATRIOT Act, which expanded surveillance powers and sparked ongoing debates about the balance between national security and civil liberties.
