September 11 Attacks
Al-Qaeda hijackers flew airliners into New York's World Trade Center and the Pentagon near Washington.
Robert J. Fisch (Flickr: TheMachineStops), edited by upstate NYer · CC BY-SA 2.0 · Source
Al-Qaeda, the Islamist extremist organisation led by Osama bin Laden, targeted the United States in protest at the presence of US troops in Saudi Arabia after the Gulf War and at American policy in the Middle East. It kept training camps in Afghanistan under the protection of the Taliban regime.
On the morning of 11 September 2001 nineteen terrorists hijacked four American airliners. Two struck the twin towers of New York’s World Trade Center, which collapsed, and one hit the Pentagon near Washington. The fourth crashed into a field in Pennsylvania as passengers fought back. About 3,000 people were killed.
The United States declared a "war on terror", attacking Afghanistan in October 2001 and toppling the Taliban, and in 2003 invaded Iraq, citing weapons of mass destruction. Airport security and surveillance were greatly tightened, and conflict with terrorist networks rather than between states became a major issue in international politics. South Korea also sent troops to Afghanistan and Iraq.