Unimaginable as it may seem, it could be worse the next time. It could be nuclear or biological terror. While we mourn our loved ones, friends, neighbors, and countrymen, we must focus on the fact that these evil people will kill as many of us as they can—unless we stop them or kill them first.

Stopping them must be our national government's overriding preoccupation for many years to come. Every one of us is a potential target in this war. A nuclear device "not much more than twice the size of this table," a national security expert told me over lunch one day as we looked out on the White House, "could level Washington from here past the Beltway, in all directions." And in parts of the Middle East, they would celebrate.

Hunting down the orchestrators of the recent horrors is one vital project. But even if we get them all, our hydra-headed enemies will threaten to strike anew at any time—dormant for years, perhaps, but deadly for decades. The long-term tasks will be protecting ourselves and our way of life from that threat, while carrying this war to enemy territory. The necessary security measures will strain our freedoms. We must maintain the vitality of those freedoms by avoiding unnecessary security measures and by not letting the garrison-state mentality, which may now be appropriate for airports and some public buildings, spill into the rest of our lives.

Privacy will shrink. Boarding an airliner will be more like boarding El Al Israel: passenger-by-passenger interrogations; intrusive luggage and, sometimes, body searches; long delays; armed guards, armed sky marshals, maybe even armed flight attendants or pilots. But even the tightest security net will have holes, so we passengers had better start thinking more like possible combatants; let this be the last time anybody brandishing knives and box cutters—box cutters!—can use an airliner as a bomb to commit mass murder. The passengers and crew who were herded to the back of the plane that crashed into the Pentagon could not have imagined what the hijackers had planned. Never again.

We must become more like Israel in other ways, for we are in this together. We face the same enemies and the same threats, and we cannot hide from them. The military and law enforcement will need to scrutinize as a potential nuclear bomb every oceangoing ship and boat that sails into every harbor in this country—and every truck headed for downtown Washington or New York or Los Angeles or Chicago. The authorities obviously cannot search them all. But they will need to search a lot of them. And they should be free to do so if they have any plausible basis for suspicion.

To ferret out evidence of terrorist conspiracies, intelligence agencies will need to comb through our communications—overseas phone calls, e-mail, Internet browsing—more intrusively than ever before. The FBI (at home) and CIA (abroad) will need to plant more informants among groups suspected of terrorist connections. "You need a government with the ability to wiretap these guys, with greater intelligence capabilities and surveillance of all kinds, with more protection for genuine national security secrets, with all of the things we think of as the enemy of our liberties—you need it to protect our liberties," says Philip Bobbitt, who was head of strategic planning on the Clinton National Security Council and now teaches constitutional law and civil liberties at the University of Texas (Austin).

"When, under conditions of modern warfare, our shores are threatened by hostile forces," the Supreme Court said in 1944, "the power to protect must be commensurate with the threatened danger." That's the right principle. But that particular decision—Korematsu v. United States, upholding the World War II internment of Japanese-Americans—has become infamous as a capitulation to racist panic in the name of military necessity. As Justice Robert H. Jackson said in dissent: "A military commander"—or, one might add, a law enforcement officer—"may overstep the bounds of constitutionality, and it is an incident. But if we review and approve, that passing incident becomes the doctrine of the Constitution. There it has a generative power of its own, and all that it creates will be in its own image."

So amid vigilant new security and surveillance, we must work with consummate care to limit the damage to our freedoms. More-intensive searching for terrorists and their weapons, for example, does not warrant more-intrusive searching for drugs, pornography, stolen goods, or even (outside of airports and public buildings) guns. Congress, law enforcement, the intelligence agencies, and the courts must refine their rules for distinguishing necessary from unnecessary intrusions into our privacy—and the courts must enforce the rules, sometimes by barring in ordinary criminal trials evidence found in counterterrorism searches that prove to be overly broad, or based on pretexts, or unjustifiably focused on people of Arab descent.

The indiscrimination of the war on drugs has done more damage to our liberties than can be justified by any good it has done. We must wage the war on terrorism both more energetically and in a more careful and discriminating way. There is nothing to be gained and much to be lost by trampling civil liberties at home just to look like we are doing something.

And much of what needs to be done need not tread on the rights of Americans. To pick just one example, the National Security Agency's ability to detect terrorist conspiracies through its worldwide electronic eavesdropping program has been eroded by lean budgets, advancing encryption technology, and the hostility of European governments, some of which are threatening to block the agency's activities in their countries. It will take money, know-how, and tough—very tough—diplomacy to reverse such damage.

We also need to rethink our strategy of pursuing known terrorist killers in hostile territory mainly through the drawn-out, uncertain channels of the legal process, and of shunning retaliatory or pre-emptive killings that might be deemed a violation of the 1976 executive order barring "assassination" by the U.S. government. "We know who a lot of these people are, and we know where they are," says a former high-level national security official. The Clinton Administration tried to kill Osama bin Laden on August 20, 1998, by hitting his terrorist training camp at Khost, Afghanistan, with cruise missiles. It did not try hard enough. Now we should finish that job.

The assassination ban was declared by President Ford in the wake of disclosures of botched CIA assassination attempts on Fidel Castro of Cuba, Patrice Lumumba of the Congo, and others during the 1960s. These were clearly unjustified violations of international law: None of the targets had sponsored attacks on the United States or its citizens. But the U.N. charter recognized that nations possess an "inherent right of individual or collective self-defense if an armed attack occurs." The President and Congress must make it unmistakably clear that when arrest and trial are impossible, we will defend ourselves by killing those who wage war on us wherever we find them, and that nations harboring terrorists have put themselves in the war zone. You want a jihad? You pay the price.

Openly targeting terrorists abroad with missiles, bombs, and (if necessary) commandos would be a grave step—but there is nothing immoral about killing in self-defense. It would risk making U.S. leaders prime targets and spawning more radical Islamic terrorists, but we cannot wage war without risking casualties.

"No one can guarantee success in war, but only deserve it," Winston Churchill said. This long, perilous struggle will test our generation as never before. We must combine pervasive surveillance with vigilant protection of privacy, lethal violence with restraint in weighing evidence and choosing targets. "For five centuries," writes Philip Bobbitt in The Shield of Achilles, a sweeping historical study of war and the constitutional order to be published in April, "it has taken the resources of the state to destroy another state ... and every state knew its enemy would be drawn from a small class of potential adversaries. This is no longer true, due to our increased interconnectedness, new technologies ... and weapons of mass destruction. The change in statecraft that will result will be as profound as any the state has undergone." The change is upon us, although we cannot foresee exactly where it will lead.

"Under what seemed to be a local dust cloud," John Hersey wrote in 1946 in Hiroshima, "the day grew darker and darker." We have all just witnessed our nation's darkest day—darker than Gettysburg, deadlier than Pearl Harbor. We must make sure it does not lead to another Hiroshima, this time on American soil. Or to another Korematsu.

We must also honor our dead and dispel the darkness by proving ourselves worthy of our nation's enduring mission: to be the last, best hope for a more civilized world.

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