Don't panic. Fear is al-Qaeda's real goal.

In responding to the attempted bombing of an airliner on Christmas Day, Sen. Dianne Feinstein voiced the feelings of many when she said that to prevent such situations, "I'd rather overreact than underreact." This appears to be the consensus view in Washington, but it is quite wrong. The purpose of terrorism is to provoke an overreaction. Its real aim is not to kill the hundreds of people directly targeted but to sow fear in the rest of the population. Terrorism is an unusual military tactic in that it depends on the response of the onlookers. If we are not terrorized, then the attack didn't work. Alas, this one worked very well.

The attempted bombing says more about al-Qaeda's weakened state than its strength. In the eight years before Sept. 11, al-Qaeda was able to launch large-scale terrorist attacks on several continents. It targeted important symbols of American power -- embassies in Africa; a naval destroyer, the USS Cole; and, of course, the World Trade Center. The operations were complex -- a simultaneous bombing of two embassies in different countries -- and involved dozens of people of different nationalities who trained around the world, moved significant sums of money and coordinated their efforts over months, sometimes years.

On Christmas an al-Qaeda affiliate launched an operation using one person, with no special target, and a failed technique tried eight years ago by "shoe bomber" Richard Reid. The plot seems to have been an opportunity that the group seized rather than the result of a well-considered strategic plan. A Nigerian fanatic with (what appeared to be) a clean background volunteered for service; he was wired up with a makeshift explosive and put on a plane. His mission failed entirely, killing not a single person. The suicide bomber was not even able to commit suicide. But al-Qaeda succeeded in its real aim, which was to throw the American system into turmoil. That's why the terror group proudly boasted about the success of its mission.

Is there some sensible reaction between panic and passivity? Philip Zelikow, the executive director of the 9/11 Commission and later a senior State Department official in the Bush administration, suggests that we should try to analyze failures in homeland security the way we do airplane catastrophes. When an airliner suffers an accident, major or minor, the National Transportation Safety Board convenes a group of nonpartisan experts who methodically examine what went wrong and then issue recommendations to improve the situation. "We approach airline security with the understanding that it's a complex problem, that we have a pretty good system, but that there will be failures -- caused by human beings, technology, or other factors. The point is to constantly fix what's broken and keep improving the design and execution," says Zelikow.