On September 11, 2001, 2,977 people were killed in the deadliest terrorist attacks in American history. The moment shocked the nation. Two planes, hijacked by Islamic jihadists vowing death to all Americans, plowed into both towers at the World Trade Center in New York. Another plane was flown into the Pentagon in Washington, DC. A fourth plane, presumably headed for the White House or the U.S. Capitol, was heroically diverted by passengers and ended up crashing in an empty field in Pennsylvania. After reports of the first plane hitting the North Tower, millions watched the second plane hit the South Tower on live television.

It was a terrifying, startling, and humbling event for the country. The 9/11 attacks were the deadliest on American soil since the shock attack at Pearl Harbor 60 years before, and the sense of outrage was reminiscent of that moment. The attacks in New York occurred in the country’s busiest city on a busy workday. And the staggered nature of the attacks meant that news footage captured almost everything as it happened, ensuring that millions of Americans saw the events precisely as they unfolded.

The plot

The September 11 attacks were precipitated in large part because Osama bin Laden, the leader of the militant Islamic organization al-Qaeda, held naive beliefs about the United States in the run-up to the attacks. Abu Walid al-Masri, an Egyptian who was a bin Laden associate in Afghanistan in the 1980s and ’90s, explained that, in the years prior to the attacks, bin Laden became increasingly convinced that America was weak. “He believed that the United States was much weaker than some of those around him thought,” Masri remembered, and “as evidence he referred to what happened to the United States in Beirut when the bombing of the Marines base led them to flee from Lebanon,” referring to the destruction of the marine barracks there in 1983 (see 1983 Beirut barracks bombings), which killed 241 American servicemen. Bin Laden believed that the United States was a “paper tiger,” a belief shaped not just by America’s departure from Lebanon following the marine barracks bombing but also by the withdrawal of American forces from Somalia in 1993, following the deaths of 18 U.S. servicemen in Mogadishu, and the American pullout from Vietnam in the 1970s.

The key operational planner of the September 11 attacks was Khalid Sheikh Mohammed (often referred to simply as “KSM” in the later 9/11 Commission Report and in the media), who had spent his youth in Kuwait. Khalid Sheikh Mohammed became active in the Muslim Brotherhood, which he joined at age 16, and then he went to the United States to attend college, receiving a degree from North Carolina Agricultural and Technical State University in 1986. Afterward he traveled to Pakistan and then Afghanistan to wage jihad against the Soviet Union, which had launched an invasion against Afghanistan in 1979. According to Yosri Fouda, a journalist at the Arabic-language cable television channel Al Jazeera who interviewed him in 2002, Khalid Sheikh Mohammed planned to blow up some dozen American planes in Asia during the mid-1990s, a plot (known as “Bojinka”) that failed, “but the dream of Khalid Sheikh Mohammed never faded. And I think by putting his hand in the hands of bin Laden, he realized that now he stood a chance of bringing about his long awaited dream.”

In 1996 Khalid Sheikh Mohammed met bin Laden in Tora Bora, Afghanistan. The 9-11 Commission (formally the National Commission on Terrorist Attacks Upon the United States), set up in 2002 by Pres. George W. Bush and the U.S. Congress to investigate the attacks of 2001, explained that it was then that Khalid Sheikh Mohammed “presented a proposal for an operation that would involve training pilots who would crash planes into buildings in the United States.” Khalid Sheikh Mohammed dreamed up the tactical innovation of using hijacked planes to attack the United States, al-Qaeda provided the personnel, money, and logistical support to execute the operation, and bin Laden wove the attacks on New York and Washington into a larger strategic framework of attacking the “far enemy”—the United States—in order to bring about regime change across the Middle East.

The September 11 plot demonstrated that al-Qaeda was an organization of global reach. The plot played out across the globe with planning meetings in Malaysia, operatives taking flight lessons in the United States, coordination by plot leaders based in Hamburg, Germany, money transfers from Dubai, and recruitment of suicide operatives from countries around the Middle East—all activities that were ultimately overseen by al-Qaeda’s leaders in Afghanistan.

Key parts of the September 11 plot took shape in Hamburg. Four of the key pilots and planners in the “Hamburg cell” who would take operational control of the September 11 attacks, including the lead hijacker Mohammed Atta, had a chance meeting on a train in Germany in 1999 with an Islamist militant who struck up a conversation with them about fighting jihad in the Russian republic of Chechnya. The militant put the Hamburg cell in touch with an al-Qaeda operative living in Germany who explained that it was difficult to get to Chechnya at that time because many travelers were being detained in Georgia. He recommended they go to Afghanistan instead.

Although Afghanistan was critical to the rise of al-Qaeda, it was the experience that some of the plotters acquired in the West that made them simultaneously more zealous and better equipped to carry out the attacks. Three of the four plotters who would pilot the hijacked planes on September 11 and one of the key planners, Ramzi Binalshibh, became more radical while living in Hamburg. Some combination of perceived or real discrimination, alienation, and homesickness seems to have turned them all in a more militant direction. Increasingly cutting themselves off from the outside world, they gradually radicalized each other, and eventually the friends decided to wage battle in bin Laden’s global jihad, setting off for Afghanistan in 1999 in search of al-Qaeda.

Atta and the other members of the Hamburg group arrived in Afghanistan in 1999 right at the moment that the September 11 plot was beginning to take shape. Bin Laden and his military commander Muhammad Atef realized that Atta and his fellow Western-educated jihadists were far better suited to lead the attacks on Washington and New York than the men they had already recruited, leading bin Laden to appoint Atta to head the operation.

The hijackers, most of whom were from Saudi Arabia, established themselves in the United States, many well in advance of the attacks. They traveled in small groups, and some of them received commercial flight training.

Throughout his stay in the United States, Atta kept Binalshibh updated on the plot’s progress via e-mail. To cloak his activities, Atta wrote the messages as if he were writing to his girlfriend “Jenny,” using innocuous code to inform Binalshibh that they were almost complete in their training and readiness for the attacks. Atta wrote in one message, “The first semester commences in three weeks…Nineteen certificates for private education and four exams.” The referenced 19 “certificates” were code that identified the 19 al-Qaeda hijackers, while the four “exams” identified the targets of the attacks.

In the early morning of August 29, 2001, Atta called Binalshibh and said he had a riddle that he was trying to solve: “Two sticks, a dash and a cake with a stick down—what is it?” After considering the question, Binalshibh realized that Atta was telling him that the attacks would occur in two weeks—the two sticks being the number 11 and the cake with a stick down a 9. Putting it together, it meant that the attacks would occur on 11-9, or 11 September (in most countries the day precedes the month in numeric dates, but in the United States the month precedes the day; hence, it was 9-11 in the United States). On September 5 Binalshibh left Germany for Pakistan. Once there he sent a messenger to Afghanistan to inform bin Laden about both the day of the attack and its scope.

The attacks

On September 11, 2001, groups of attackers boarded four domestic aircraft at three East Coast airports, and soon after takeoff they disabled the crews, some of whom may have been stabbed with box cutters the hijackers were secreting. The hijackers then took control of the aircraft, all large and bound for the West Coast with full loads of fuel. At 8:46 AM the first plane, American Airlines flight 11, which had originated from Boston, was piloted into the north tower of the World Trade Center in New York City. Most observers construed this initially to be an accident involving a small commuter plane. The second plane, United Airlines flight 175, also from Boston, struck the south tower 17 minutes later. At this point there was no doubt that the United States was under attack. Each structure was badly damaged by the impact and erupted into flames. Office workers who were trapped above the points of impact in some cases leapt to their deaths rather than face the infernos now raging inside the towers. The third plane, American Airlines flight 77, taking off from Dulles Airport near Washington, D.C., struck the southwest side of the Pentagon (just outside the city) at 9:37 AM, touching off a fire in that section of the structure. Minutes later the Federal Aviation Authority ordered a nationwide ground stop, and within the next hour (at 10:03 AM) the fourth aircraft, United Airlines flight 93 from Newark, New Jersey, crashed near Shanksville in the Pennsylvania countryside after its passengers—informed of events via cellular phone—attempted to overpower their assailants.

At 9:59 AM the World Trade Center’s heavily damaged south tower collapsed, and the north tower fell 29 minutes later. Clouds of smoke and debris quickly filled the streets of Lower Manhattan. Office workers and residents ran in panic as they tried to outpace the billowing debris clouds. A number of other buildings adjacent to the twin towers suffered serious damage, and several subsequently fell. Fires at the World Trade Center site smoldered for more than three months. Rescue operations began almost immediately as the country and the world sought to come to grips with the enormity of the losses. Nearly 3,000 people had perished: some 2,750 people in New York, 184 at the Pentagon, and 40 in Pennsylvania; all 19 terrorists also died. Included in the total in New York City were more than 400 police officers and firefighters, who had lost their lives after rushing to the scene and into the towers.

On the morning of September 11, President Bush had been visiting a second-grade classroom in Sarasota, Florida, when he was informed that a plane had flown into the World Trade Center. A little later Andrew Card, his chief of staff, whispered in the president’s right ear: “A second plane hit the second tower. America is under attack.” To keep the president out of harm’s way, Bush subsequently hopscotched across the country on Air Force One, landing in Washington, D.C., the evening of the attacks. At 8:30 PM Bush addressed the nation from the Oval Office in a speech that laid out a key doctrine of his administration’s future foreign policy: “We will make no distinction between the terrorists who committed these acts and those who harbor them.”

On September 14 Bush visited “Ground Zero,” the smoking pile of debris of what remained of the World Trade Center and the thousands who had perished there. Standing on top of a wrecked fire truck, Bush grabbed a bullhorn to address the rescue workers working feverishly to find any survivors. When one of the workers said that he could not hear what the president was saying, Bush made one of the most memorable remarks of his presidency:

I can hear you. The rest of the world hears you. And the people who knocked these buildings down will hear from all of us soon. Bush’s robust response to the attacks drove his poll ratings from 55 percent favourable before September 11 to 90 percent in the days after, the highest ever recorded for a president.

The aftermath

The emotional distress caused by the attacks—particularly the collapse of the twin towers, New York City’s most visible landmark—was overwhelming. Unlike the relatively isolated site of the Pearl Harbor attack of 1941, to which the September 11 events were soon compared, the World Trade Center lay at the heart of one of the world’s largest cities. Hundreds of thousands of people witnessed the attacks firsthand (many onlookers photographed events or recorded them with video cameras), and millions watched the tragedy unfold live on television. In the days that followed September 11, the footage of the attacks was replayed in the media countless times, as were the scenes of throngs of people, stricken with grief, gathering at “Ground Zero”—as the site where the towers once stood came to be commonly known—some with photos of missing loved ones, seeking some hint of their fate.

Moreover, world markets were badly shaken. The towers were at the heart of New York’s financial district, and damage to Lower Manhattan’s infrastructure, combined with fears of stock market panic, kept New York markets closed for four trading days. Markets afterward suffered record losses. The attacks also stranded tens of thousands of people throughout the United States, as U.S. airspace remained closed for commercial aviation until September 13, and normal service, with more rigid security measures, did not resume for several days.

The hunt for bin Laden

In September 2001 President Bush announced that he wanted Osama bin Laden captured—dead or alive—and a $25 million bounty was eventually issued for information leading to the killing or capture of bin Laden. Bin Laden evaded capture, however, including in December 2001, when he was tracked by U.S. forces to the mountains of Tora Bora in eastern Afghanistan. Bin Laden’s trail subsequently went cold, and he was thought to be living somewhere in the Afghanistan-Pakistan tribal regions. U.S. intelligence eventually located him in Pakistan, living in the garrison city of Abbottabad, and in the early morning hours of May 2, 2011, on orders from U.S. Pres. Barack Obama, a small team of U.S. Navy SEALs assaulted his compound and shot and killed the al-Qaeda leader.