The term functions as a proper noun, a specific name for an event. It refers to a series of four coordinated terrorist attacks by the Islamic extremist group al-Qaeda against the United States on the morning of Tuesday, September 11, 2001. The incidents were the deadliest terrorist attacks in world history, instigating major U.S. initiatives to combat terrorism and defining the foreign policy of the subsequent decade.
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 of the World Trade Center complex in New York City, leading to the collapse of both skyscrapers. 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 passengers and crew attempted to regain control from the hijackers. The total number of fatalities was 2,977, not including the 19 perpetrators.
The immediate consequence of these events was the initiation of the U.S. "War on Terror," which began with the invasion of Afghanistan to depose the Taliban, who had harbored al-Qaeda. Domestically, the U.S. government enacted significant policy changes, including the creation of the Department of Homeland Security and the passage of the USA PATRIOT Act, which expanded surveillance powers. Globally, the incidents led to a dramatic increase in aviation security measures and fundamentally altered international relations and national security paradigms worldwide.
