(FORTUNE Magazine) – I was living in London in 1994, when the Irish Republican Army declared its first cease-fire. Naturally, after 25 years of intermittent outrages, people were wary of the promise. For weeks after the announcement, life was not significantly different. People still looked at you incredulously if you asked them to watch your bag. Libraries still kept their book-return slots closed; they were vulnerable to bomb attacks. Only after months with no new atrocities did daily life grow easier and more pleasant.

The following year I was visiting Paris, whose rail and subway stations had for the past few months been devastated by terrorist hits. All the garbage bins, which had been used to hide bombs, were shuttered. Parisians with litter placed it neatly under the bins, the mounds of rubbish making a distinctive and smelly trail. A small thing, perhaps, but disturbing to a people and country that take pride in their public places.

Now it is America's turn to react to terrorism. In one blow, the U.S. has almost certainly suffered more casualties than Northern Ireland has experienced in 32 years of its wretched troubles. This was not, of course, only an attack on Americans. Some three million New Yorkers were born in another country, and the World Trade Center was as polyglot as any place on earth. Whoever the terrorists are, their slaughter of innocents is certain to have included people of their own race, religion, or nation.

There are two challenges now. One is to deliver us from the evil people who did this. The second is subtler. It is to not change our society in ways that will ultimately damage it.

There will, no question, be some loss of personal liberty in the wake of this tragedy. Air travel will never be the same; security checks will be more stringent, time-consuming, and invasive. That is a price we will pay every time we board a plane or perhaps even enter an airport.

But let's carefully consider such costs before we incur them. America has a bad habit of taking a tragedy, like Columbine, and then going overboard to combat it, such as instituting zero-tolerance policies for nursery school kids roughhousing. There is going to be a temptation, in the wake of this disaster, to treat every prominent building or area as a fortress under permanent, dire threat. It is easy enough to imagine pressure for, say, metal detectors in state parks, national identity cards, limiting the public's access to official places, bulletproof shields in post offices, or extraordinary powers for the police or military. But turning America into an armed and fearful camp in response to the events of Sept. 11 would simply give those who wish us ill another victory.

The price of liberty, the saying goes, is eternal vigilance. That surely means vigilance against the kind of people who would fly planes into crowded skyscrapers. But it also means vigilance against exercising excessive zeal in our own determination not to let such terror happen again.

The human cost of this terrorism is unfathomable; the economic cost, incalculable. But we do have the power, and the responsibility, to determine the cost to our freedom.