Grammatically, the term "911 attack" functions as a compound noun. In this phrase, "911" (an abbreviation for the date, September 11th) serves as a noun adjunct that modifies the head noun "attack." This specifies the unique historical event being referenced: the series of four coordinated terrorist attacks by the Islamic extremist group al-Qaeda against the United States on the morning of September 11, 2001.
The attacks were executed by 19 al-Qaeda terrorists who hijacked four commercial airliners. Two of the planes, American Airlines Flight 11 and United Airlines Flight 175, were crashed into the North and South Towers, respectively, of the World Trade Center in New York City, leading to their eventual collapse. A third plane, American Airlines Flight 77, was crashed into the Pentagon in Arlington, Virginia. The fourth plane, United Airlines Flight 93, crashed into a field in Shanksville, Pennsylvania, after its passengers and crew fought back against the hijackers. The total number of fatalities from the attacks was 2,977, including the 19 perpetrators.
The consequences of the event fundamentally altered U.S. foreign and domestic policy. It directly triggered the multi-decade global "War on Terror," which included the invasion of Afghanistan to dismantle al-Qaeda and depose the Taliban. Domestically, it prompted the creation of the Department of Homeland Security and the passage of the USA PATRIOT Act, legislation that significantly expanded surveillance capabilities. The attacks also led to sweeping changes in aviation security worldwide and have had a lasting impact on international relations, civil liberties, and national security protocols.
