The September 11 attacks (called September 11, September 11th or 9/11),were a series of four coordinated suicide attacksagainst targets in New York and Washington, D.C. on September 11, 2001. On that morning, 19 al-Qaeda terrorists hijacked four passenger jets. The hijackers intentionally crashed two planes into the Twin Towers of the World Trade Center in New York City; both towers collapsed within two hours. Hijackers crashed a third plane into the Pentagon in Arlington, Virginia. When passengers attempted to take control of the fourth plane, United Airlines Flight 93, it crashed into a field near Shanksville, Pennsylvania, preventing it from reaching its intended target in Washington, D.C. Nearly 3,000 died in the attacks.

Suspicion quickly fell on al-Qaeda, the Islamist militant group. In 2004, Al-Qaeda leader Osama bin Laden, who had initially denied involvement, later claimed responsibility for the attacks. Al-Qaeda and bin Laden cited U.S. support of Israel, the presence of U.S. troops in Saudi Arabia, and sanctions against Iraq as motives for the attacks. The United States responded to the attacks by launching the War on Terror, invading Afghanistan to depose the Taliban, who had harbored al-Qaeda members. Many countries strengthened their anti-terrorism legislation and expanded law enforcement powers. In May 2011 bin Laden was found and killed. It is also noted that in the ten day period before the attacks, Bin Laden had been widening public admonitions of US Middle East policies, seemingly laying the ground for making all Americans a terrorist target.

The destruction caused serious damage to the economy of Lower Manhattan.Cleanup of the World Trade Center site was completed in May 2002. The National September 11 Memorial & Museum is scheduled to open on September 11, 2011. Adjacent to the memorial the 1,776 feet (541 m) One World Trade Center is estimated for completion by 2013. The Pentagon was repaired within a year, and thePentagon Memorial opened, adjacent to the building, in 2008. Ground was broken for the Flight 93 National Memorial in November 2009, and the memorial is to be formally dedicated on September 10, 2011.