What event does September 11th, 2001 refer to?

September 11th, 2001 is recognized primarily for the series of terrorist attacks carried out by the extremist group al-Qaeda. On that day, four commercial airliners were hijacked, with two planes crashing into the Twin Towers of the World Trade Center in New York City, one into the Pentagon in Arlington, Virginia, and the fourth, United Airlines Flight 93, crashing into a field in Pennsylvania after passengers attempted to regain control from the hijackers. This event had profound implications for U.S. domestic and foreign policy, leading to significant changes in national security, a global war on terror, and subsequent military actions in various regions. Understanding this date is crucial for comprehending the modern geopolitical landscape and the resulting impacts on military operations and strategy.