September 11 refers to the date in 2001 when a series of coordinated terrorist attacks were carried out by the extremist group al-Qaeda against the United States, resulting in the destruction of the Twin Towers of the World Trade Center in New York City and significant damage to the Pentagon in Arlington, Virginia. This pivotal event fundamentally changed U.S. foreign and domestic policies, leading to a global 'War on Terror' and significant shifts in national security strategies.