The day after New York's World Trade Center was leveled in a terrorist attack, firefighters raised a defiant American flag on the smoking rubble of the twin towers. Across the country, millions of people have followed suit, with flags on their homes, in shop windows, or high in their hands. They fly the flags to honor the victims and show their shared determination to overcome the worst act of terrorism ever committed on U.S. soil.

Thousands died during the attack of September 11. That morning, terrorists hijacked four commercial aircraft. They crashed two of the planes into the World Trade Center towers. A third plowed into the Pentagon, headquarters of the U.S. military, near Washington, D.C. The fourth crashed in Pennsylvania, apparently after passengers fought back against the hijackers.

Who attacked the U.S. and why? How could this have happened? We may never know all the facts. But here are some questions and answers about what happened and what might happen now.

Q: What is terrorism?
Terrorism is the use of violence to scare people. The goal of most terrorist groups is to force their victims to change policies or actions.

Terrorists commonly operate in small, secret organizations. Such groups are not big enough to defeat a military power like the U.S. head-on, so they use surprise, hit-and-run, and even suicide attacks to do as much damage as possible. "It's a strategy that allows the weak to attack the strong," says Harvard Professor Arnold M. Howitt, an expert on how to prepare for terrorist attacks. As a rule, terrorists strike high-profile targets, such as the World Trade Center and the Pentagon, to bring as much attention to their cause as possible.

Q: Who were the terrorists who attacked the U.S.?
The U.S. government has launched a massive in- vestigation to learn who carried out the September 11 attacks. Early clues indicate that the men who hijacked the jets came from Arab countries in the Middle East.

Certain Arab groups, including some fanatical followers of Islam, harbor hatred of the U.S. and what it stands for. "They resent that the U.S. is a strong supporter of Israel," says Harvard Professor Philip B. Heymann, a terrorism expert. And they feel the U.S. has betrayed Palestinian Arabs in their struggles with Israel. In addition, says Heymann, these extreme groups see American values—such as diversity, tolerance, freedom of expression, and freedom of religion—as evils that they want destroyed and kept out of their own societies.

Q: Why couldn't the U.S. intelligence agencies stop the attack?
The terrorists gave no warning before the action. U.S. intelligence agencies, including the Federal Bureau of Investigation (FBI) and the Central Intelligence Agency (CIA), are good at sniffing out and stopping many terrorist plots. But they can't know everything, and can't simply arrest every suspect who they fear might commit a crime.

Now these agencies are using all their resources to figure out who was involved, how it happened, where the agencies failed, and how to prevent similar attacks in the future.

Q: What is being done to guard against future terrorist attacks?
The U.S. government is beefing up security, especially at airports. Baggage will be searched more thoroughly, and passengers will no longer be able to carry knives aboard planes. Putting an armed air marshal on each flight also has been suggested, as well as constructing cockpits so that hijackers cannot gain entry.

Experts are also increasing security at U.S. borders and ports. They want to make it tougher for terrorists to sneak into the country or smuggle in weapons.

Q: What will the U.S. do to punish those responsible?
The U.S. government and the American people are determined to bring those responsible to justice. The hijackers, of course, were killed on board the planes. But law enforcement officials are certain that dozens, perhaps hundreds, of people helped them. "Make no mistake," said President George W. Bush, "the United States will hunt down and punish those responsible for these cowardly acts."

That trail will probably lead overseas. Countries like Afghanistan, Pakistan, Iran, and Iraq have a reputation for protecting and supporting terrorist organizations. If the U.S. finds people or organizations in these countries that helped with the September 11 plot, it is likely the U.S. will mount a military attack to try to capture or destroy them. It will not be easy. As President George W. Bush said, "This will be a different type of war than we're used to. It's an enemy that likes to hide and burrow and their network is extensive. . . . [but] we're going to smoke them out."

Even so, U.S. leaders realize they must be sure to strike the right people, or the retaliation could create more enemies among the countries of the world.

Q: How will this terrorist act change our country?
Security will increase to try to prevent future terrorist acts. You may see more police or guards patrolling at public events, like concerts, football games, and next year's Winter Olympics. At airports, you may have to stand in long lines as airline staff search passengers' baggage.

You may also notice people being friendlier and more helpful to each other. Terrible events often have a way of bonding people together, even if they are complete strangers.

Q: Why are some Americans blaming Arab-Americans and Muslims?
Evidence suggests that at least some of the hijackers were Muslim extremists from Arab countries. This has led a few rash and unthinking Americans to hate and want to punish all Muslims and Arabs. Mobs have attacked mosques—Muslim places of worship. Individuals also have been attacked.

To return hate with hate is the worst thing Americans can do. "Don't let the hate that led to terrorism spread," says Dr. Bruce Perry, an expert in helping families and children deal with tragedy. "This was the work of a small group of hateful people. Blame them, not some kid in your class who is different from you."

Q: Why do people do such terrible things?
Some people or groups are so filled with hate that it can make them crazy. If it goes far enough, they can even stop respecting human life. This is especially true if they believe their victims are evil, dangerous, or less than human. That's why hate is not an emotion to toy with. It can poison peoples' minds until they become evil.

Q: How can we help?
Along with people's thoughts and prayers, money is the main need of the victims and their families. You and your classmates could hold a fundraiser and make a donation. Check with local charities, like the Red Cross.

Closer to home, support people in your own family, class, school, and community. This was an attack on all Americans. Many of us—even far from where the attacks occurred—are scared, worried, and angry. We must all come together, wherever we are, to get through this and to rebuild.