The worst international terrorist attack ever - involving four separate but coordinated aircraft hijackings - occurred in the United States on September 11, 2001. The 19 hijackers belonged to the al-Qaida terrorist network. According to investigators and records of cellular phone calls made by passengers aboard the planes, the hijackers used knives and boxcutters to kill or wound passengers and the pilots, and then commandeer the aircraft, which the hijackers used to destroy preselected targets.

• Five terrorists hijacked American Airlines flight 11, which departed Boston for Los Angeles at 7:45 a.m. An hour later it was deliberately piloted into the North Tower of the World Trade Center in New York City.
• Five terrorists hijacked United Airlines flight 175, which departed Boston for Los Angeles at 7:58 a.m. At 9:05 the plane crashed into the South Tower of the World Trade Center. Both towers collapsed shortly thereafter, killing approximately 3000 persons, including hundreds of firefighters and rescue personnel who were helping to evacuate the buildings.
• Four terrorists hijacked United Airlines flight 93, which departed Newark for San Francisco at 8:01 a.m. At 10:10 the plane crashed in Stony Creek Township, Pennsylvania killing all 45 persons on board. The intended target of this hijacked plane is not known, but it is believed that passengers overpowered the terrorists, thus preventing the aircraft from being used as a missile.
• Five terrorists hijacked American Airlines flight 77, which departed Washington Dulles Airport for Los Angeles at 8:10 a.m. At 9:39 the plane was flown directly into the Pentagon in Arlington, Virginia, near Washington, D.C. A total of 189 persons were killed, including all who were onboard the plane.

More than 3000 persons were killed in these four attacks. Citizens of 78 countries perished at the World Trade Center site. "Freedom and democracy are under attack," said President Bush the following day. Leaders from around the world called the events of September 11 an attack on civilization itself.

The coordinated attack was an act of war against the United States. President Bush said in a 20 September 2001 address to a joint session of Congress: "Our war on terror begins with al-Qaida, but it does not end there. It will not end until every terrorist group of global reach has been found, stopped, and defeated."

Virtually every nation condemned the attack and joined the US-led Coalition to fight terror on several fronts: diplomatic, economic, intelligence, law enforcement, and military. Operation Enduring Freedom, the military component of the Coalition, began on 7 October. The first targets were the al-Qaida training camps and military installations of the Taliban regime in Afghanistan. Islamic extremists from around the world - including North America, Europe, Africa, the Middle East, and Central, South, and Southeast Asia - had used Afghanistan as a training ground and base of operations for worldwide terrorist activities.

Within months, the Taliban was driven from power, and nearly 1000 al-Qaida operatives were arrested in over 60 countries.

At year's end, the war continued to be waged on all fronts and was certain to last well into the future.