In Europe modern terror has literally got under our skins: our onboard radar system scans for the shopping bag left behind under a bus seat; our peripheral vision watches for the subtlest signs of weird or furtive behavior in airport lounges, hotel lobbies and subway stations. Every time we step out of the house, our nervous system is programmed for basic risk assessment--which airline to fly, which airport to use, which baggage handler to trust, which route to take--and while all this has slipped into the realm of the taken for granted, our paranoia threshold drops lower by the month.

Since Baader-Meinhof in Germany, the Red Brigades in Italy and the I.R.A. in the United Kingdom, Europeans have had 25 years to retune their nervous system. In Europe the bomb has become banal: even the voice on the public address system ordering you to evacuate a London subway station sounds bored. Now it's America's turn to enter the brave new world of the permanent security alert.

But the American nervous system is still in sleep mode, in comparison to the European. British police consultants were scathing about Olympic security before the Atlanta bomb, and afterward, when British news crews were combing through Centennial Olympic Park, they zeroed in on what, to a European eye, is an astounding sight: uncovered trash cans. There hasn't been an uncovered trash can in a Paris street for a year; there isn't one to be found in a British airport or railway station. Security cleared them away a long time ago.

Terror lowers the threshold of paranoia; it also lowers the threshold of liberty. There is barely a whisper of a debate in Europe about the astounding intrusiveness of state security. During the trial of an Islamic terrorist in Paris recently, French security effectively shut down the center of the city for a week. European travelers don't bother to look up from their laptops when an armored car cruises across an airport runway; the traders and dealers of London's financial district don't seem to mind that closed-circuit cameras photograph both driver and license plate on every car heading to work through the City's so-called ring of steel. It's hard to imagine Wall Street brokers putting up with equivalent intrusion.

Americans seem much more sensitive to the civil liberties implications of security than most Europeans. When the British Parliament rushed through a set of new stop-and- search powers for the police in the wake of the renewed I.R.A. bombing campaign earlier this year, British civil libertarians struggled in vain to fan the smoke of debate into a decent flame. After 25 years of terror in Britain, there are more closed-circuit TV surveillance systems at work per capita than anywhere else in the world. British police chiefs candidly admit that terrorism has licensed a level of security intrusion into ordinary people's lives that would be unthinkable across the Atlantic.

It would be a delusion to suppose that this intrusion is licensed by active consent. True, people don't mind being stopped and searched if they know it's for their own protection. The problem is that body searches and closed-circuit TV are just the tip of the security iceberg. Below the waterline is the archipelago of wiretap, infiltration and covert surveillance. All of Europe's cold-war security services have retooled for terrorism, and their operatives have been infiltrating everywhere, from Kurdish workers' hostels in Germany and Algerian mosques in Marseilles to Irish pubs in London. It is doubtful that the weary public consensus that sustains this covert campaign actually has the faintest idea how far the tentacles of the European state reach into everyone's lives. That may be terrorism's greatest victory.

As the President and Congress marshal the American debate on the response to TWA 800, the Centennial Olympic Park bombing and Oklahoma City, the lesson from Europe seems plain: if you don't confront the civil liberties implications right away, if you don't debate the trade-offs between security and liberty from the outset, you won't have anything to debate later on. Because the machine will be in place and it will be largely invisible. It would be a fine irony if in responding to the threat from terrorists, the Administration ended up giving ordinary Americans more reason to feel paranoid about the Federal Government.

The real problem is as old as political theory itself: who guards the guardians? Who runs the security check on the security checkers? How do we make sure that the defense of liberal society doesn't end in destroying what we want to defend? The real civil liberties' issues are not about baggage checks and intrusive passenger questioning, but about how far we should turn our secret warriors loose on the enemies of society. As the British found in Northern Ireland, the strategies that work--infiltration, covert operations, counterterror--are also the strategies that are hardest to control.

Canadian historian Michael Ignatieff's most recent book is Blood and Belonging: Journeys into the New Nationalism.