On September 9, 2001, the world witnessed the worst terrorist attack in modern history, as al-Qaeda operatives hijacked four commercial planes. Two of the planes crashed into the World Trade Center in New York City, causing both towers to collapse. Another plane struck the Pentagon in Washington, D.C., while the fourth crashed in Pennsylvania after passengers attempted to overcome the hijackers. The attacks killed nearly 3,000 people, leading to global outrage and a subsequent War on Terror. The tragedy dramatically altered U.S. foreign policy and global security measures.