In January 2001, the American economy was strong, George W. Bush had just become the country's 43rd president, and people were optimistic about the future. But nine months later, on September 11, 2001, the United States suffered the worst terrorist attack in its history. (Terrorism is the use of violence to intimidate people with different political or social views than your own.) Americans' confidence quickly turned into shock, fear, and anger.

Early on the morning of September 11, terrorists from a radical Muslim group called al-Qaeda hijacked four commercial flights and attempted to fly them into landmarks on the East Coast. Three of the flights succeeded in hitting their targets, killing thousands in the planes and on the ground. The day and the attacks themselves are usually called "9/11."

For more detail about what happened, click through the slideshow.

The First Attack The first two hijacked flights--American Airlines Flight 11 and United Airlines Flight 175--were commandeered shortly after takeoff from Boston's Logan Airport. Between 8:45 and 9:00 a.m., both planes were flown into the Twin Towers of the World Trade Center in New York City. This picture shows the moment when the second plane struck a tower. Both Towers Destroyed The passengers, crew, and hijackers were all killed instantly when the planes struck the towers, and the intense fires weakened the towers' support. Within two hours, both of the 110-story towers had collapsed on themselves. Nearly 2,600 people died, and thousands more were wounded. Flight Path As you can see, both flights took off from Boston and were used as suicide projectiles within an hour of leaving Logan Airport. Second Attack While two planes were striking the World Trade Center, other terrorists hijacked American Airlines Flight 77 and flew it into the west wing of the Pentagon, the headquarters of the Department of Defense just outside Washington, D.C. A total of 189 people were killed, including 65 on the plane. Flight Path The flight that struck the Pentagon had taken off from Dulles International Airport outside Washington, D.C. It was hijacked and turned around 30 minutes into the flight.

Only an hour passed between the first hijacking and the destruction of the first three planes. Americans were shocked by news that an explosion had occurred at the World Trade Center, and it wasn't until the second plane struck the towers on live TV that everyone realized that the United States was under attack.