On September 11, 2001, terrorists hijacked, or took control of, four airplanes in the United States. They flew two of the planes into the twin towers of the World Trade Center in New York City, causing the towers to catch fire and collapse. Another plane destroyed part of the Pentagon building (U.S. military headquarters) in Arlington, Virginia. The fourth plane crashed in Shanksville, Pennsylvania. All told, nearly 3,000 people were killed in the attacks. The events of September 11 (often called "9/11") have been described as the worst-ever terrorist attacks against the U.S.

Investigations revealed that the hijackings were carried out by 19 men — five on each of the first three planes and four on the last. All 19 were reportedly linked to the terrorist group Al Qaeda (ahl KAY-dah). The group's leader, Osama bin Laden, operated out of Afghanistan. The U.S. government identified bin Laden as the main planner of the attacks.

It was not the first time that bin Laden was held responsible for attacks on the U.S. The U.S. government implicated him in the earlier World Trade Center bombing of 1993 and in the 1998 bombings of the U.S. embassies in Kenya and Tanzania.

In response to the September 11 attacks, U.S. President George W. Bush proclaimed a war on terrorism. Bush demanded that the Taliban, the Islamic group that ran Afghanistan's government, hand over bin Laden. The Taliban refused. On October 7, 2001, the United States began air strikes on targets in Afghanistan connected to bin Laden and Al Qaeda. That launched the Afghanistan War. U.S.-led forces soon toppled the Taliban and helped build a more democratic government in Afghanistan.

About 100,000 U.S. troops are now serving in Afghanistan. More than 2,100 U.S. soldiers have been killed there since 2001. Since the war began, many top Al Qaeda leaders have been captured or killed. But until recently, bin Laden could not be found. He was suspected of hiding out in the mountains near the border of Pakistan.

On May 1, 2011, President Barack Obama announced that U.S. military forces located and killed Osama bin laden in Pakistan.