On Tuesday morning, a piece was torn out of our world. A patch of blue sky that should not have been there opened up in the New York skyline. In my neighborhood—I live eight blocks from the World Trade Center—the heavens were raining human beings. Our city was changed forever. Our country was changed forever. Our world was changed forever.

It will take months merely to know what happened, far longer to feel so much grief, longer still to understand its meaning. It's already clear, however, that one aspect of the catastrophe is of supreme importance for the future: the danger of the use of weapons of mass destruction, and especially the use of nuclear weapons. This danger includes their use by a terrorist group but is by no means restricted to it. It is part of a larger danger that has been for the most part ignored since the end of the cold war.

Among the small number who have been concerned with nuclear arms in recent years—they have pretty much all known one another by their first names—it was commonly heard that the world would not return its attention to this subject until a nuclear weapon was again set off somewhere in the world. Then, the tiny club said to itself, the world would awaken to its danger. Many of the ingredients of the catastrophe were obvious. The repeated suicide-homicides of the bombers in Israel made it obvious that there were people so possessed by their cause that, in an exaltation of hatred, they would do anything in its name. Many reports—most recently an article in The New York Times on the very morning of the attack—reminded the public that the world was awash in nuclear materials and the wherewithal for other weapons of mass destruction. Russia is bursting at the seams with these materials. The suicide bombers and the market in nuclear materials was that two-plus-two that points toward the proverbial necessary four. But history is a trickster. The fates came up with a horror that was unforeseen. No one had identified the civilian airliner as a weapon of mass destruction, but it occurred to the diabolical imagination of those who conceived Tuesday's attack that it could be one. The invention illumined the nature of terrorism in modern times. These terrorists carried no bombs—only knives, if initial reports are to be believed. In short, they turned the tremendous forces inherent in modern technical society—in this case, Boeing 767s brimming with jet fuel—against itself.

So it is also with the more commonly recognized weapons of mass destruction. Their materials can be built the hard way, from scratch, as Iraq came within an ace of doing until stopped by the Gulf War and as Pakistan and India have done, or they can be diverted from Russian, or for that matter American or English or French or Chinese, stockpiles. In the one case, it is nuclear know-how that is turned against its inventors, in the other it is their hardware. Either way, it is "blowback"—the use of a technical capacity against its creator—and, as such, represents the pronounced suicidal tendencies of modern society.

This suicidal bent—nicely captured in the name of the still current nuclear policy "mutual assured destruction"—of course exists in forms even more devastating than possible terrorist attacks. India and Pakistan, which both possess nuclear weapons and have recently engaged in one of their many hot wars, are the likeliest candidates. Most important—and most forgotten—are the some 30,000 nuclear weapons that remain in the arsenals of Russia and the United States. The Bush Administration has announced its intention of breaking out of the antiballistic missile treaty of 1972, which bans antinuclear defenses, and the Russians have answered that if this treaty is abandoned the whole framework of nuclear arms control built up over thirty years may collapse. There is no quarrel between the United States and Russia that suggests a nuclear exchange between them, but accidents are another matter, and, as Tuesday's attack has shown, the mood and even the structure of the international order can change overnight.

What should be done? Should the terrorists who carried out Tuesday's attacks be brought to justice and punished, as the President wants to do? Of course. Who should be punished if not people who would hurl a cargo of innocent human beings against a fixed target of other innocent human beings? (When weighing the efficiency—as distinct from the satisfaction—of punishment, however, it is well to remember that the immediate attackers have administered the supposed supreme punishment of death to themselves.) Should further steps be taken to protect the country and the world from terrorism, including nuclear terrorism? They should. And yet even as we do these things, we must hold, as if to life itself, to a fundamental truth that has been known to all thoughtful people since the destruction of Hiroshima: There is no technical solution to the vulnerability of modern populations to weapons of mass destruction. After the attack, Secretary of Defense Rumsfeld placed US forces on the highest state of alert and ordered destroyers and aircraft carriers to take up positions up and down the coasts of the United States. But none of these measures can repeal the vulnerability of modern society to its own inventions, revealed by that heart-breaking gap in the New York skyline. This, obviously, holds equally true for that other Maginot line, the proposed system of national missile defense. Thirty billion dollars is being spent on intelligence annually. We can assume that some portion of that was devoted to protecting the World Trade Center after it was first bombed in 1993. There may have been mistakes—maybe we'll find out—but the truth is that no one on earth can demonstrate that the expenditure of even ten times that amount can prevent a terrorist attack on the United States or any other country. The combination of the extraordinary power of modern technology, the universal and instantaneous spread of information in the information age and the mobility inherent in a globalized economy prevents it.

Man, however, is not merely a technical animal. Aristotle pointed out that we are also a political animal, and it is to politics that we must return for the solutions that hold promise. That means returning to the treaties that the United States has recently been discarding like so much old newspaper—the one dealing, for example, with an International Criminal Court (useful for tracking down terrorists and bringing them to justice), with global warming and, above all, of course, with nuclear arms and the other weapons of mass destruction, biological and chemical. The United States and seven other countries now rely for their national security on the retaliatory execution of destruction a millionfold greater than the Tuesday attacks. The exit from this folly, by which we endanger ourselves as much as others, must be found. Rediscovering ourselves as political animals also means understanding the sources of the hatred that the United States has incurred in a decade of neglect and, worse, neglect of international affairs—a task that is highly unwelcome to many in current circumstances but nevertheless is indispensable to the future safety of the United States and the world.

It would be disrespectful of the dead to in any way minimize the catastrophe that has overtaken New York. Yet at the same time we must keep room in our minds for the fact that it could have been worse. To lose two huge buildings and the people in them is one thing; to lose all of Manhattan—or much, much more—is another. The emptiness in the sky can spread. We have been warned.