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Your teachers and parents undoubtedly all have a story about September 11, 2001. They probably remember where they were when they first saw an image of American Flight 11 or United Flight 175 striking the World Trade Center. For those who lived through the 9/11 attacks, the images remain a touchstone of our lives. But if you’re an American high-school student today, the attacks of 9/11 probably happened before you were born. Your country has been at war your entire life.

On the morning of September 11, 2001, 19 men from the terrorist organization Al-Qaeda hijacked four passenger airplanes. They flew two of the planes into the World Trade Center towers in New York City and one into the Pentagon. A fourth plane heading to Washington, DC crashed in a Pennsylvanian field. The attackers killed 2,977 people.

This wasn’t their first attack on the United States. During the 1990s, Al-Qaeda had launched other, smaller attacks on American military targets and embassies abroad. In 2001, Osama bin Laden—the Al-Qaeda leader who planned the attacks—was living in Afghanistan under the protection of the Taliban. The Taliban are a conservative Islamist political and military group that ruled most of Afghanistan in 2001. Following 9/11, U.S. President George W. Bush demanded the Taliban surrender Osama bin Laden. When they refused, an American-led coalition invaded Afghanistan in October 2001, and US troops remain there today, in the spring of 2020.

The 9/11 attacks killed thousands. The wars and policies that followed reshaped American foreign and domestic policy and set in motion a global war on terror. Why? What caused these attacks?

Why do they hate us?

These were the questions on the minds of millions of Americans in the fall of 2001: Why do they hate us? And who were “they”? In the wake of the 9/11 attacks, many Americans turned their attention to the most obvious difference between themselves and the attackers: religion. News reports were filled with terms like “Islamic extremism” and “Islamism.” Many Americans knew little about Islam, the Muslim religion, let alone the radical forms of Islamist thought followed by a tiny fraction of the world’s 1.5 billion Muslims. One of the best-selling world history textbooks in America during 1990s dealt with Islam in only one chapter, on the eleventh to thirteenth century conflicts known as the crusades. So, many American students had encountered Islam only as a religion that existed 500 years ago. After the attacks, sales of the Islamic holy book, the Quran, soared. It was easy, in a climate of fear and anger, to blame a whole religion for the attacks.

The idea that the attackers (and Muslims more generally) hated America because they hated Americans’ “western- style” freedom was comforting to some people. It provided simple answers. The argument went something like this: “Islam does not share Western values. They hate us because we’re a free society.” But blaming an entire religion of 1.5 billion people for the events of 9/11—committed by a much smaller group of extremists—wasn’t an accurate response. There are, and were in 2001, many American Muslims, including members of the armed forces, willing to put their lives on the line for the United States. Islam is a diverse religion encompassing many ideas, from secular to spiritual to deeply religious. Islamism—the notion that Islam should guide personal life and society at large—has many peaceful practitioners.

Why do we hate them?

The oversimplified notion that 9/11 was a result of a war between Islam and the West had deep roots in American intellectual thought in 2001. In the 1990s, several scholars tried to explain how global politics would change in the new millennium. Political scientist Samuel Huntington wrote about “The Clash of Civilizations.” He argued that “the great divisions among humankind and the dominating sources of conflict will be cultural.” Huntington divided the world into civilizations guided by different ideologies. Now that the Cold War was over, Huntington believed that Western civilization must clash with Islamic civilization. In the months after the attacks of 9/11, Huntington’s book joined the Quran on best-seller lists. It was very popular in the George W. Bush (2001-2009) White House.

As Americans began to recover from the shock of the 9/11 attacks, a disturbing trend emerged. According to the FBI, hate crimes against Muslims surged. On September 17, President Bush called Islam a religion of peace. But the day before, he had announced a global war on terror, calling it a “crusade.” The crusades were a series of medieval wars, in which Christian European armies invaded Muslim states. Bush’s word choice was significant because it framed the conflict in civilizational terms.

New enemies and old conflicts

Unlike the “clash of civilizations” idea, more accurate explanations of what caused the 9/11 attacks acknowledge historical complexity. European colonialism in the Middle East during the nineteenth and twentieth centuries still influences politics in the region. Even after colonialism, many people in the Middle East saw interference from the United States and Soviet Union as a new kind of imperialism. The Soviet Union occupied Afghanistan in 1979 and fought a bloody guerilla war through most of the 1980s. The United States funded and armed the guerrillas, many of whom—including Osama bin Laden—would eventually join Al-Qaeda.

After the Soviet Union collapsed in 1991, the United States seemed on top of the world as the lone superpower. In 1991, the U.S. military led a large coalition, including many Islamic states, in a highly successful Gulf War campaign against Iraqi President Saddam Hussein. But from bin Laden’s perspective, the fall of the Soviet Union and the Gulf War provided very different lessons: Superpowers can fall. Bin Laden wanted the remaining superpower to fall as well. During the Gulf War, half a million international troops were stationed in Saudi Arabia. Saudi Arabia is the home of the holy city of Mecca, however. For a religious extremist like bin Laden, the presence of foreign troops in the land of Mecca was an insult to Islam. He began to hate both the United States and the Muslim leaders who were its allies in the Gulf War.

Bin Laden and his organization began targeting Americans with terror attacks soon after the Gulf War. These attacks were part of a wider strategy. They believed that by attacking America, they could provoke the U.S. government into a disproportionate response. This would turn public opinion in the Islamic world against the Americans. That in turn would lead to the downfall of the moderate and secular governments in the Islamic world, allowing bin Laden and his allies to create an extremist, religious state in the Middle East.

Forever war

The United States did react to the events of 9/11, of course, but the results were not quite as bin Laden had imagined them. In October 2001, the American military and dozens of allies invaded Afghanistan. The coalition quickly defeated the Taliban in what the U.S. military codenamed “Operation Enduring Freedom.” But this was a global war on terror, and for the Bush administration, the conflict was just beginning. The president placed terrorist groups like Al-Qaeda in an “Axis of Evil” that included unfriendly states like Saddam Hussein’s Iraq. However, Iraq had not in fact been involved in the events of 9/11. By incorrectly connecting terrorism to Iraq, the government created justification for another war. President Bush made his views clear, saying: “either you are with us or you are with the terrorists.” The Americans and a smaller coalition of allies invaded Iraq in 2003 against the wishes of the United Nations, falsely claiming that Saddam Hussein was building weapons of mass destruction.

The invasion of Iraq was part of a campaign to combat terrorism by overthrowing unfriendly leaders and replacing them with American-installed democratic governments. But the results of this strategy were a disaster. Despite broad public support in the U.S. for the invasion of Iraq and the downfall of Saddam Hussein, the war dragged on. Iraq quickly plunged into civil war, and the American invasion increased recruitment for militant groups in the Middle East, as bin Laden had hoped. However, the US maintained the support of most of its Muslim allies in the region. American special forces killed Osama bin Laden in 2011, during President Obama’s first term in office.

The killing of bin Laden did not end the global war on terror. By 2011, new threats and new leaders had sprung up around the region. The most infamous of these called itself the Islamic State (ISIS). In Afghanistan, the Taliban was overthrown in 2001, but in 2020, the United States is once again seeking negotiations with the Taliban to end the conflict.

The cost

In the United States, two decades of war have transformed domestic life. Government measures like the Patriot Act and the Homeland Security Department have increased government powers on the borders, in the airports, and in private life. The war in Afghanistan recently surpassed the Vietnam War as America’s longest conflict. Funding these wars has changed the American economy. In the chart below you’ll see that, in 2014, the U.S. government spent more money on their military than the next nine nations combined, adding to the national debt.

How do we calculate the cost of 9/11? Almost 3,000 people died in the attacks. Thousands of soldiers have died in wars in Afghanistan, Pakistan, Iraq, and Syria. But by far, the largest group of casualties have been civilians. Estimates differ, but we know hundreds of thousands of civilians have died in these wars. In the region more generally, bin Laden got what he wanted. A massive American military occupation of several Middle Eastern nations has generated hostility to American presence and destabilized several governments in the Middle East, doing lasting damage to America’s influence around the world.