Suppose for a moment that the harmless Lite-Brites that threw Boston into such pandemonium last week hadn't been so harmless after all. Suppose that the 38 illuminated devices attached to highway overpasses and other public spots around the city hadn't been "guerrilla art" intended to promote an animated show on cable TV, but the terrorist bombs that authorities at first feared they were. Suppose the individuals behind this operation in Boston and nine other cities had been devotees not of Aqua Teen Hunger Force, an inane cartoon about talking fast food, but of Al Qaeda and its violent, totalitarian version of Islam.

Suppose the worst had very nearly come to pass, and had been averted only by the grace of God and the nick-of-time intervention of the police department bomb squads. What would we be doing now? Patting ourselves on the back for winning a round in the war against terrorism? Hardly. We would be gasping at how close we had just come to suffering a devastating attack.

In the wake of last week's bomb scare, public discussion seemed to divide into two camps: those who were enraged at the perpetrators of the stunt and the massive chaos is led to, and those who mocked city officials for overreacting hysterically to something many younger residents knew at once was a marketing gimmick. But the police weren't wrong not to take any chances; even before 9/11, thousands of people around the world wound up in early graves because something that appeared to be innocuous -- a suitcase, a toy, a man's bulky coat, a yellow Ryder rental truck -- had turned out to be a terrorist's bomb.

Still: If public safety depends on a timely and effective police response to the appearance of every suspicious object, the public had better not count on being very safe. Spotting a bomb in time to defuse it is the *last* line of defense against a terrorist attack -- the one we're left with when everything else has failed, or when nothing else has been done.

Sharp-eyed passersby calling 911 will never be numerous enough to flag every anomaly that might be hiding a bomb. The most adroit and agile sappers can never be 100 percent sure that a lethal booby-trap isn't ticking somewhere, unnoticed. However skilled first responders and security officials are at reacting to dangerous *things,* it is not the things themselves that pose the greatest danger to us in the war against militant Islam. It is the people behind those things, and the radical jihadist ideology that motivates them. We cannot be secure unless we pre-empt such people before they can act, and discredit that ideology before it poisons new minds.

Vast resources were marshaled in Boston last week to address what turned out to be a nonexistent threat. "At the peak of the alert," Reuters noted, "authorities mobilized emergency crews, federal agents, bomb squads, hundreds of police and the US Coast Guard. . . . Roads, bridges, and even part of the Charles River were closed." A stunning amount of manpower, time, and money was thrown at the mere possibility of a "danger" that no one had even known about a day before. But what about the dangers that we know are only too real? How much energy and expense do local authorities devote to monitoring the circles in which radical Islamists indoctrinate and recruit their followers? Are state and local government pulling out all the stops to expose and counter the jihadist message that we know can transform peaceful Muslims into implacable Islamists?

It is too easy to focus government attention on specific objects -- shoes and liquids at the airport, knives and metal objects at the entrance to public buildings, mysterious Lite-Brites on the undersides of bridges. It is tougher to keep a sustained focus on human beings who share certain beliefs, a form of surveillance from which most Americans instinctively recoil. Ideological and religious profiling goes against our civil-liberties grain. Infiltrating Islamic groups, keeping tabs on mosques, applying heightened scrutiny to Muslims in order to track the extremists among them -- we tend to find such activities distasteful, awkward, even un-American.

But if we intend to win the war the jihadists have declared against us, they are unavoidable. The chaos in Boston last week was absurd and expensive and truly much ado about nothing. But it was also a warning: Societies at war cannot wait for bombs to be phoned in to 911. We must stop the Islamists before they strike. That in turn means knowing who they are, what they say, and where they are. Even if we would rather not.