On September 11, 2001, members of the Islamic extremist group al-Qaeda hijacked four commercial airplanes to carry out suicide attacks against the United States. Two planes struck the Twin Towers of the World Trade Center in New York City, a third hit the Pentagon in Arlington, Virginia, and a fourth crashed in a field in Shanksville, Pennsylvania, after passengers fought back. The attacks killed nearly 3,000 people and triggered the global 'War on Terror.' This event led to massive changes in international security, the creation of the Department of Homeland Security, and the U.S.-led invasions of Afghanistan and Iraq. It remains a defining moment of the early 21st century, reshaping global geopolitics, civil liberties, and the way the world approaches counter-terrorism and aviation safety.