The September 11 attacks, commonly known as 9/11, were four coordinated suicide terrorist attacks carried out by the militant Islamic extremist network al-Qaeda against the United States on Tuesday, September 11, 2001. That morning, nineteen terrorists hijacked four commercial airliners scheduled to travel from the northeastern U.S. to California. The hijackers crashed the first two planes into the Twin Towers of the World Trade Center in New York City, and the third plane into the Pentagon (the headquarters of the American military) in Arlington, Virginia. The fourth plane was intended to hit a federal government building in Washington, D.C., but crashed in a field following a passenger revolt. The attacks killed nearly 3,000 people and instigated the global war on terror.

The first impact was that of American Airlines Flight 11 at 8:46 am, into the North Tower of the World Trade Center complex in Lower Manhattan. Seventeen minutes later, at 9:03 am, the World Trade Center's South Tower was hit by United Airlines Flight 175. Both 110-story towers collapsed within an hour and forty-two minutes, precipitating the collapse of other World Trade Center structures including 7 World Trade Center, and damaging nearby buildings. A third flight, American Airlines Flight 77, crashed into the west side of the Pentagon in Arlington County, Virginia, at 9:37 am, causing a partial collapse. The fourth and final flight, United Airlines Flight 93, flew in the direction of Washington, D.C. Alerted of the previous attacks, the plane's passengers attempted to regain control, but the hijackers ultimately crashed the plane in a field in Stonycreek Township, Pennsylvania, near Shanksville, at 10:03 am. Investigators determined that Flight 93 was targeting either the U.S. Capitol or the White House.

Within hours of the attacks, suspicion quickly fell onto al-Qaeda. The United States formally responded by launching the war on terror and invading Afghanistan to depose the Taliban, which had not complied with U.S. demands to expel al-Qaeda from Afghanistan and extradite its leader, Osama bin Laden. The U.S.'s invocation of Article 5 of NATOits only usage to datecalled upon allies to fight al-Qaeda. As U.S. and NATO ground forces swept through Afghanistan, bin Laden fled to the White Mountains, where he narrowly avoided capture by U.S.-led forces. Although bin Laden initially denied any involvement, in 2004 he formally claimed responsibility for the attacks. Al-Qaeda's cited motivations included U.S. support of Israel, the presence of U.S. troops in Saudi Arabia, and sanctions against Iraq. After evading capture for almost a decade, bin Laden was killed by the U.S. military on May 2, 2011.

The attacks resulted in 2,977 non-hijacker fatalities, an indeterminate number of injuries, and substantial long-term health consequences, in addition to at least $10 billion in infrastructure and property damage. It remains the deadliest terrorist attack in human history and the single deadliest incident for firefighters and law enforcement officers in U.S. history, with 340 and 72 killed, respectively. The destruction of the World Trade Center and its environs seriously harmed the New York City economy and induced global market shocks. Many other countries strengthened anti-terrorism legislation and expanded their powers of law enforcement and intelligence agencies. Cleanup of the World Trade Center site (colloquially "Ground Zero") took eight months and was completed in May 2002, while the Pentagon was repaired within a year. After delays in the design of a replacement complex, the One World Trade Center began construction in November 2006 and opened in November 2014. Memorials to the attacks include the National September 11 Memorial & Museum in New York City, the Pentagon Memorial in Arlington County, Virginia, and the Flight 93 National Memorial at the Pennsylvania crash site.

The New York Stock Exchange (NYSE, nicknamed "The Big Board") is an American stock exchange in the Financial District of Lower Manhattan in New York City. It is by far the world's largest stock exchange by market capitalization of its listed companies at US$30.1 trillion as of February 2018. The average daily trading value was approximately US$169 billion in 2013. The NYSE trading floor is at the New York Stock Exchange Building on 11 Wall Street and 18 Broad Street and is a National Historic Landmark. An additional trading room, at 30 Broad Street, was closed in February 2007.

The NYSE is owned by Intercontinental Exchange, an American holding company that it also lists (NYSE: ICE). Previously, it was part of NYSE Euronext (NYX), which was formed by the NYSE's 2007 merger with Euronext.