ON DAY 18 of the war in Iraq, a single United States Air Force B-1 bomber attacked a residence in the north of Baghdad where Saddam Hussein was believed to be hiding. The effects were dramatic. Explosions not only demolished the structure entirely but left a gigantic crater of jumbled steel and debris 60 feet deep and 150 feet wide. This devastation was caused by four conventional bombs, each weighing 2,000 pounds. They are by no means the heaviest bombs in the U.S. arsenal. The Air Force’s “Daisy Cutter” weighs in at 15,000 pounds and can dig a much deeper and wider area of destruction.

But these devices, fearsome though they may be, are trivial in their effects compared with a nuclear weapon. If the destructive power of each of the bombs dropped in Baghdad was roughly equivalent to 1,000 pounds of TNT (trinitrotoluene), a nuclear bomb fueled by a single pound of a fissionable element like uranium or plutonium would release the explosive equivalent of approximately sixteen million pounds (eight kilotons). Over the course of the nuclear age, devices in the megaton range (millions of tons of TNT) have been developed and tested.

The tremendous force of a nuclear blast causes correspondingly greater destruction, including from its sheer heat. Whereas a conventional explosion generates temperatures nearing 9,000 degrees Fahrenheit, a nuclear detonation unleashes heat in the millions of degrees, which is then dispersed with terrible effect. In the initial phase, all of the material of the bomb itself—the nuclear fuel, the metal casing, the triggering device—is converted instantaneously into an intensely compressed vapor. Within less than a millionth of a second, this vapor expands into a highly luminous mass of burning air and nuclear material that ascends on its own far up into the atmosphere, reaching widths as large as thousands of feet across.

On the surface of the earth, the fireball vaporizes whatever solid materials abut the explosion, including soil and rock, which then fuse with the radioactive elements of the bomb itself and are borne aloft, gradually returning to earth as fallout: highly lethal radioactive particles ranging in width from the size of a grain of fine sand to small marbles. The rapidly expanding gas of the explosion also gives off a shock wave, a wall of air that continues to move away from the explosive center well after the fireball has disappeared. The wave generates winds exceeding several hundred miles an hour at the epicenter of the explosion and can cause destruction for miles around.

Finally, nuclear weapons yield radiation, including highly penetrating gamma rays that remain lethal over a considerable distance. The rays from a one-megaton explosion can extend approximately two miles; at one mile from ground zero, one would need a concrete barrier four-feet thick to afford protection from them—on the unlikely assumption one could survive the blast’s other, more violent effects.

Nuclear weapons have been used in anger only twice: first at Hiroshima on August 6, 1945, and then three days later, when the Japanese still refused to capitulate, at Nagasaki. In all, the immediate death toll from the two attacks was approximately 150,000, with many more tens of thousands left gravely injured. Whatever one’s view of President Truman’s decision to employ the bomb against Japan, no one then or later would dispute that these are the most dreadful weapons ever devised.

Which is why, ever since their invention, a mainstay of American policy has been to prevent a surprise attack with them on our soil. During the cold war, one main leg of this effort was the policy of deterrence, aimed at convincing our principal adversary, the USSR, that a nuclear strike on the U.S. would be met by an even more devastating counterattack that would wipe the USSR from the map. The policy worked, and now that the Soviet empire is no more, we are engaged in a largely cooperative relationship with its nuclear- and non-nuclear-armed successor states.

A second leg of our effort was, and still is, aimed at keeping nuclear weapons from falling into the wrong hands. Until relatively recently, this policy too has largely been a success. Here, technology was long on our side. So considerable were the costs and expertise required to create nuclear weapons that, in the first decades after World War II, only highly developed countries—the USSR, China, England, and France (and, by the late 1960’s, perhaps Israel)—succeeded in developing them on their own. But with the passage of years, the spread of civilian nuclear technology—especially nuclear power plants—and the emergence of a global cadre of nuclear engineers and physicists steadily reduced the obstacles to building such weapons. The essentials of bomb design are today widely understood, and key technologies can either be fabricated indigenously or purchased on open or black and gray markets. Only the nuclear fuel itself—plutonium or highly enriched uranium—remains exceedingly difficult to acquire, although countries with civilian nuclear-power programs can create it on their own.

The U.S. has employed a variety of diplomatic instruments to prevent the spread of nuclear weapons. The primary tool—the “cornerstone of U.S. nuclear nonproliferation policies,” according to a ranking Bush administration official—is the Treaty on the Non-Proliferation of Nuclear Weapons (NPT). This multilateral agreement became international law in 1970 and has by now been signed by some 187 nations—all the nations of the world save three: India, Pakistan, and Israel.*

Along with lofty-sounding provisions calling for peace, the elimination of all nuclear weapons from the planet, and a number of other general goals, the NPT includes a number of specific measures. In particular, it obligates those signatories who do not already have nuclear weapons to remain in that condition, and to accept regular inspections by the International Atomic Energy Agency (IAEA) to verify that any civilian nuclear resources are under “safeguard” and are not being covertly diverted to military ends.

In some respects, the NPT has worked extremely well. Thanks to IAEA inspections, the U.S. government and the world community have access to a wealth of highly detailed information about the civilian nuclear programs of countries around the globe, including countries hostile to the United States. The NPT regime has also played a vital role in preserving the nuclear-free status of regional rivals like Argentina and Brazil, to name two countries that in the 1970’s and 80’s were veering into a nuclear-arms race. Perhaps the treaty’s most remarkable achievement was to have fostered the denuclearization of South Africa; as F. W. de Klerk, that country’s former president, would confess, South Africa had surreptitiously developed a small nuclear arsenal, but then dismantled and destroyed it in order to accede to the agreement in 1991.

Such accomplishments have led supporters of the NPT to insist, in the words of the Bush administration, that the “global nuclear nonproliferation regime remains strong.” But the global nuclear nonproliferation regime is not strong. It has been in serious and growing difficulty for years, and is now virtually in tatters. The story of its decline is full of the most worrisome implications for the future course of world politics. It is also a case study in the pitfalls of relying on multilateral arms-control agreements to protect critical U.S. interests.

IN RECENT years, the NPT regime has faced serious challenge from four countries, and flunked each test. In the case of only one of them—Iraq—has the crisis been definitively resolved, but at the cost of two major wars. Other dangers remain very much upon us, and they are both terrible to contemplate and difficult to avoid.

The history of Iraq’s nuclear program exemplifies what has gone wrong. Iraq ratified the NPT in 1969 under Saddam Hussein, but the country’s signature was an act of deceit. From the outset, the Iraqi dictator was seeking to acquire nuclear weapons; by the mid-1970’s, assisted by avid European suppliers, he had an active program under way. By 1981, Iraqi scientists were on the verge of gaining access to a plentiful source of nuclear fuel from their new reactor at Osirak, a turn-key facility provided by France. Then, on June 7, 1981, Israel, fearing a nuclear-armed Saddam in its neighborhood, destroyed the facility in a precision air-strike that shocked the world.

Iraq responded to this setback by reconstituting its secret program, dispersing facilities widely and placing key technology in hardened shelters. Although the program’s existence was widely suspected, IAEA inspectors came and went without uncovering evidence that radioactive materials were either being diverted from civilian reactors or being acquired by other means. Only in 1991, in the aftermath of the first Gulf war, did the scope and scale of Iraq’s prewar efforts become evident.

Yet even in defeat, and even after having signed surrender terms pledging to disclose all information about the illicit program, Saddam Hussein’s government continued to engage in denial and deception. At first it stated flatly that it had “no industrial and support facilities related to any form of atomic-energy use which have to be declared.” When this statement was rebutted with incontrovertible facts by the IAEA, the regime acknowledged a handful of sites but still failed to disclose the lion’s share of its activity. Only after the IAEA initiated special on-site inspections did Iraq begin to release significant information, even then omitting important details and either blocking IAEA access to key sites or hurriedly removing nuclear-related equipment from locations that inspectors were likely to visit. The full scope of the Iraqi effort become evident only when the IAEA stumbled on a trove of classified documents.

Under the noses of IAEA inspectors, those documents revealed, the Iraqis had constructed what Hans Blix, then the head of the agency, ruefully admitted was a “vast unknown, undeclared uranium-enrichment program in the billion-dollar range,” constituting an essential part of “an advanced nuclear-weapons development program.” Among other things, Iraq was in possession of some 400 tons of previously undisclosed radioactive materials, including six grams of clandestinely produced plutonium and more than 35 kilograms of highly enriched uranium—not yet bomb-grade material but of “high strategic value.” Iraq had also acquired a large number of calutrons for enriching uranium; these electro-magnetic devices, used by the U.S. in constructing its first atomic bombs but subsequently abandoned in favor of more efficient means, were extremely well suited for a clandestine program like Iraq’s.

It seems that, at the time of the Iraqi invasion of Kuwait in 1990, Baghdad had been only months away from acquiring a workable nuclear device. Had Saddam Hussein been a little more patient, he could have had a nuclear-equipped military before embarking on that aggressive adventure. Standing up to him in those circumstances would have presented incalculably greater risks to Washington and  its hesitant allies in Europe.

Nor, in the aftermath of the first Gulf war, did Iraq cease its activity. A great deal of information came to light in 1995 with the defection to Jordan of Hussein Kamel, Saddam Hussein’s son-in-law, who revealed a well-funded and continuing program to mount a nuclear warhead on an intermediate-range ballistic missile as well as efforts to turn highly enriched uranium into fuel for a nuclear bomb. Once again, these efforts were proceeding in the face of special IAEA and UNSCOM inspections mandated by the UN Security Council and far more intrusive than the ones for a normal country under the NPT.

What happened to Iraq’s nuclear program after the mid-decade revelations, and especially after 1998 when Saddam Hussein halted all cooperation with the UN inspectors and they withdrew from the country, is unclear. As is well known, Washington based its case for the second Gulf war in part on intelligence pointing to a continuing covert Iraqi effort to acquire nuclear weapons, including the highly controversial sixteen words in President Bush’s State of the Union Address about Iraq’s alleged effort to purchase uranium yellowcake from the African country of Niger. But in the aftermath of our victory, the search for evidence of this program has thus far come up dry. Did the Iraqi dictator order the program transferred to new and as yet undiscovered locations, or was it dismantled and destroyed? We do not yet have the answer.

IF IRAQ represents one kind of failure for the NPT, Pakistan represents another—not so much of the treaty itself as of U.S. policy. The salient fact here is that Pakistan has refused to sign the pact, and is not subject to its strictures.

The Pakistani nuclear program, like Iraq’s, is decades old. It began in earnest after the loss of East Pakistan—now Bangladesh—to India in the war of 1971, a defeat that impelled Pakistan to develop an “Islamic bomb” (in the phrase employed at the time by prime minister Zulfikar Ali Bhutto) to counter India’s Hindu one. The fuel for this Islamic bomb was initially to come from a reprocessing facility provided by France in 1974, although the French and other Western suppliers withdrew as Pakistani intentions became clear. In stepped the Chinese, who in the intervening decades have provided Pakistan with technicians, highly enriched uranium, key components of enrichment facilities, and a heavy-water reactor for the production of plutonium and tritium, as well as designs for a relatively sophisticated and readily deliverable 25-kiloton-yield weapon.

Lacking recourse to the machinery of the NPT, the U.S. has responded to this Pakistani program with an assortment of carrots and sticks, pledging financial and military assistance if Pakistan would desist, threatening a series of sanctions, some of them mandated by Congress, if it pressed ahead. But the sanctions have been waived at every turn, for the simple reason that Pakistan has been a pivotal player in U.S. foreign policy as a frontline state both in the Soviet-Afghan war that began in 1979 and in the war against al Qaeda and the Taliban that began in October 2001. In any case, the sanctions were unlikely to have deflected Pakistan from a strategic goal it has perceived as vital to its national existence.

Already by the mid-1990’s, Pakistan was widely believed to have obtained a small stockpile of nuclear weapons, as well as the missiles for delivering them. Its status as a nuclear power was confirmed when it conducted five underground tests on May 28, 1998. By any yardstick, this date deserves to be remembered as a watershed in international affairs, marking the first time that a certifiable basket-case of a country became an officially-declared nuclear power.

Since its birth as a nation in 1947, Pakistan’s government has been regularly toppled by military coups. A major segment of the population is in the grip of radical Islam, and some leading nuclear scientists have close ties to the most fanatical Muslims of Afghanistan and al Qaeda. The country is locked in a conflict with India over the status of Kashmir that periodically threatens to become the first nuclear flashpoint since World War II. To complete the picture, Pakistan is so desperately poor that it has been paying for its military programs by barter.

Its most important partner in this arrangement happens to be North Korea. In exchange for North Korean missiles that can carry a nuclear payload, Pakistan has provided Pyongyang with gas centrifuges, a key technology for processing uranium into bomb-grade material. The U.S. response to this illicit trade has been a mild slap on the wrist: this past April, Washington imposed a two-year ban on any American dealings with the research laboratory where Pakistan’s nuclear weapons are designed and fabricated.

IF PAKISTAN is a stick of dynamite, North Korea is a stick of dynamite with a lit fuse. The Democratic Peoples Republic of Korea (DPRK), run today by the mad Communist dictator Kim Jong Il, became a signatory of the NPT in 1985. But from the outset it declined to permit the IAEA to verify its initial accounting of nuclear materials, or to monitor more than a single one of its reactors. As the charade continued in the 1990’s, the Clinton administration engaged in an intense but ultimately fruitless effort to persuade the North to abandon its nuclear ambitions, encouraging it to sign a supplementary agreement—the Yongbyon Agreed Framework, brokered by former President Jimmy Carter—that promised generous foreign aid in exchange for forbearance. North Korea grudgingly accepted the aid but, as we now know, declined to show any forbearance.†

The most dramatic chapter of this saga opened last October, when for no discernible reason Pyongyang suddenly revealed that, in violation of both the NPT and Yongbyon, it was operating an active nuclear-weapons program all along. By December it had ratcheted up the pressure, declaring the Yongbyon agreement null and void and renouncing the NPT in the bargain. On New Year’s eve, all IAEA personnel were expelled from the country. In April, Pyongyang declared that it already possessed nuclear weapons and was in the midst of manufacturing more, having reprocessed the fuel from 8,000 control rods at one of its “civilian” reactors. In August, it announced that it might shortly commence test-firing nuclear weapons, something it has not yet openly done (although one of Pakistan’s nuclear tests may actually have been of a North Korean device).

The North Korean regime is Stalinist to the core—and then some. Thanks to a calcified, centrally-planned economy, large portions of the country suffer from famine. Amid the general destitution, Kim Jong Il has sponsored a personality cult whose symbols and slogans are ubiquitous. His subjects speak of him with the mandatory appellation “Dear Leader” and wear a badge of his likeness on their lapels. The North Korean regime has engaged in bizarre kidnapping plots (of South Korean actors and actresses, to jump-start an indigenous film industry; of girls off beaches in Japan, to be employed as teachers of Japanese language and manners in a school for spies). Pyongyang has also engaged in terrorism. Among other violent deeds, it blew up a South Korean airliner in 1987, killing all 115 aboard.

It is this demented and venomous regime that boasts of having nuclear weapons at its disposal. According to the CIA, in addition to the one or two bombs already in its possession, the North has been “constructing a plant that could produce enough weapons-grade uranium for two or more nuclear weapons per year when fully operational—which could be as soon as mid-decade.” According to another government study, Pyongyang has also been at work on two very large “electrical-generating” stations that, upon completion, will produce sufficient spent nuclear fuel to yield 200 kilograms of plutonium, enough to manufacture approximately 30 nuclear weapons a year.

Compounding the peril is the fact that North Korea has been vigorously developing intermediate- and long-range missiles capable of carrying nuclear warheads. It has already successfully tested intermediate-range missiles that can strike all of Japan, points far beyond in Asia and the Pacific, and—with a reduced payload—the west coast of the United States. In September, U.S. officials reported a new model in the works with a range of 9,400 miles, a capability that would place every city in the United States under its shadow.

Not only is North Korea steadily adding missiles to its own arsenal, it is exporting them to other unsavory regimes around the world. With its ample supplies of uranium and uranium-enrichment equipment, it has threatened to export nuclear materials as well. Not only does North Korea “pose a serious and immediate challenge to the nuclear non-proliferation regime,” in the words of Mohamed ElBaradei, the current head of the IAEA; it poses an even more serious and immediate challenge to the peace and security of the world.

AMONG THE countries trading with North Korea is Iran, a country likewise governed by violent fanatics, of the Islamic rather than the Marxist-Leninist stripe. Iran joined the NPT at the treaty’s inception. It was then still under the rule of the shah, who had started an ambitious civilian-nuclear program and possibly some weapons-related research as well. But IAEA inspectors were finally invited to visit the country’s facilities only in 1992, thirteen years after the shah was deposed by the Islamic revolution. The ayatollahs appear to have calculated that, being limited to officially designated sites, the IAEA would be unable to find evidence of their secret program. If so, their calculation proved correct, for the IAEA regularly certified Iran to be in compliance with the treaty’s strictures—until it became unmistakably apparent that  it had been in violation all along.

Earlier this year, in the face of detailed media reports, Iran admitted to the IAEA that it had been constructing two hitherto secret plants: one to enrich uranium and another to produce heavy water, an essential ingredient in developing plutonium. The Iranians also acknowledged having imported nearly two metric tons of uranium from China in 1991, which, in a major breach of the NPT, they stored in a facility not subject to IAEA supervision. In late August and again in late September, IAEA inspections turned up traces of uranium on equipment in supposedly non-nuclear facilities, leading the agency to conclude that an illicit enrichment program was under way. Commented ElBaradei: “This worries us greatly.”

Iran is an oil-rich country. It has no need for an ambitious civilian nuclear-energy industry. The fact that it has been vigorously developing one was a red flag that the ayatollahs did not deign to conceal. To augment the menace, Iran is “the most active state sponsor of terrorism” in the world, according to the U.S. Department of State. Tehran has carried out a series of kidnappings and assassinations in Europe. It has funded and provided training and arms to a variety of Palestinian terrorist organizations, including Hizballah, Hamas, Islamic Jihad, and factions within Yasir Arafat’s PLO. It was almost certainly behind the bombings in Argentina of the Israeli embassy in March 1992, killing 29, and the Jewish community center in July 1994, killing 86. It is thought to have had a hand in the June 1996 bombing of the al-Khobar barracks in Saudi Arabia that took the lives of nineteen U.S. soldiers. It has ties with al Qaeda and, in the wake of September 11, may have given shelter to some of its leading operatives. The list goes on and on.

To augment the menace even more, Iran has also been building missiles at a feverish pace. In July it successfully tested the Shehab-3 (a variant of the No Dong missile first provided to it by North Korea), with a range of 930 miles and capable of carrying a small nuclear warhead. Iranian engineers are similarly moving forward with the Shehab-4 and Shehab-5, with ranges of 1,240 and 3,100 miles respectively. Brigadier General Safavi, who heads Iran’s Revolutionary Guard Corps, declared not long ago that “Iranian missiles can cause irreparable damage to either Israel or the United States.” This is partly bluster. Israel indeed lies within range of Iranian missiles. The United States does not—not yet.

PERHAPS BECAUSE  the attention of our policymakers has been diverted elsewhere, perhaps because our military resources are stretched thin in Afghanistan and Iraq, perhaps because the options are all so unattractive, perhaps because the issues are so dire, the twin challenge presented by North Korea and Iran has met with an even more muted American response than has the challenge posed by Pakistan.

In Asia, the U.S. has been engaged in desultory six-way talks with North Korea and its neighbors. The idea is to bring pressure to bear on Pyongyang, especially from China and Russia, while also holding out the prospect of still more aid if North Korea dismantles its program in a verifiable way. It would be something of a miracle if the talks were to succeed; this approach has been tried in the past and failed.

Under the Yongbyon framework, the Clinton administration plied North Korea with huge shipments of oil. It also promised two proliferation-resistant light-water nuclear reactors if Pyongyang would only promise to stop developing the bomb. In a magnanimous gesture during Clinton’s final year in office, Madeleine Albright became the first American cabinet member ever to visit Communist Pyongyang—following which, noting “important progress” in talks about missile exports, the administration eased longstanding sanctions against the North under the Trading with the Enemy Act, the Defense Production Act, and the Export Administration Act. But there was no “important progress”: North Korea did not limit its missile exports or do anything else, except, presumably, absorb a lesson or two about American credulity. In the end, one may hope that it will turn out to be an incorrect lesson; but if, today, the North Koreans make preposterous demands and feign outrage when we do not yield, at least we have some inkling why.

The American reaction to developments in Iran has been even quieter. Once again, we have attempted to work in concert with neighbors and, especially, the IAEA to pressure the ayatollahs to adhere to their obligations under the NPT or face the disapprobation of the UN Security Council. The IAEA is also seeking Iran’s signature on a supplementary protocol that would make the country more “transparent” to inspectors. The success of these initiatives may be judged by the fact that Ayatollah Khatami, Iran’s “moderate” president, has pledged continued fealty to the NPT even as his regime blatantly breaches its provisions. Other influential clerics, including Ayatollah Jannati, head of the Guardian Council and closely aligned with Iran’s “hardline” supreme leader, Ayatollah Kha­menei, have urged that the government shun the “extra humiliation” of the new protocol and follow the path of North Korea by withdrawing from the NPT altogether.

HOW NORTH KOREA and Iran will conduct themselves in the months to come is a matter of speculation. Many different behaviors are possible, ranging from delaying tactics to phony concessions to threats of aggression. But, the immediate future aside, the predicament we are in is as unmistakable as is our apparent determination to ignore or deny it.

The NPT regime is radically flawed. Three countries whose facilities have been under its safeguards have managed either to develop nuclear weapons or to come perilously close to it. This has occurred because the NPT exhibits almost all the classical problems of arms-control agreements as Washington has pursued them. Elaborate mechanisms are put in place that seem to ensure the achievement of desirable objectives. Yet, in the absence of airtight verification procedures, the only countries thereby restrained are the law-abiding ones who are not themselves a menace. In the meantime, determined cheaters like Iraq, Iran, and North Korea make use of loopholes to pursue their objectives. Though the NPT appeared to work well in its early years, when the relevant technology was more difficult to acquire, now it serves mostly as a cover for would-be proliferators, offering assurances to the world that everything is fine and encouraging Washington to slumber when it needs most to be alert.

The NPT also exhibits structural defects specific unto itself. IAEA inspectors, of whom there are only several hundred responsible for policing approximately 1,000 nuclear facilities around the world, can barely do their job as it is. They are spread even thinner by the need to devote the same amount of attention to wholly innocuous programs in countries like Canada as they do to suspicious ones in countries like Iran. At the same time, IAEA officials lack the freedom to conduct unfettered inspections of any site they choose; they can only visit sites declared (by the signatory nation) to be under the IAEA’s “safeguard.” And even if they were granted more sweeping rights, the idea that they could find undeclared facilities on their own in a country attempting to conceal them is a delusion. Finally, a glaring loophole in the treaty exempts states from declaring a nuclear installation until 180 days before introducing radioactive material into it; this is precisely the escape mechanism that Iran has exploited to build the uranium and plutonium facilities it has only now disclosed.

In theory the NPT could be strengthened by closing its loopholes and mandating intrusive inspections of sites selected by the inspectors themselves. But the political obstacles would be formidable, and the countries of greatest concern would almost certainly demur. Even if there were universal agreement about amending the NPT, moreover, it would remain only as strong as the will of its strongest members to enforce it. Thus far, with the exception of the decisive action taken on two occasions by the United States against Iraq, that will has been absent. One should note, of course, that even here, Iraqi breaches of the NPT were not a casus belli cited by the U.S. before either Gulf war.

As for Pakistan, as a non-member state, it would of course not be directly touched by any changes to the NPT.  For the moment, unlike Pyongyang and Tehran, the government of Pakistan does still seem capable of making rational choices. But if that situation were to change, and radical Islamists were to ascend to power, the prospect that Pakistani nuclear weapons might be transferred to the remnants of al Qaeda or to other Islamic terrorists would be intolerable. Both India and the United States would feel under tremendous pressure to disarm Islamabad, a step that in the logic of things would quite possibly require a nuclear first strike.

U.S. influence on the future course of Pakistani politics is quite limited. Where the U.S. might play an active role right now is in making it utterly clear to our ostensible ally that unless it ceases to export its nuclear know-how and materials to rogue states, it will be made to pay a very stiff price. Similar efforts might also be made to rein in or punish other exporters of nuclear material, including not only Pakistan and North Korea but also Russia, China, and France.

THE RADICAL insufficiency of the NPT confirms once again the wisdom of deploying a missile-defense shield. This project, widely ridiculed when it was first proposed by Ronald Reagan in the 1980’s, has become an urgent national imperative. The U.S. needs a strategic system to defend its own skies, and portable ship- or air-borne theater systems to defend its allies.

But even if we could deploy an impermeable missile shield tomorrow (and no missile shield is likely to be impermeable), there are other ways than missiles to deliver nuclear weapons. Such weapons can be packed into shipping containers and brought into American ports, or smuggled across our borders wrapped inside, say, a bale of marijuana. Countering this particular facet of the threat defensively is virtually impossible—a fact that points toward yet another urgent imperative.

In the National Security Strategy he unveiled at West Point in June 2002, President Bush enunciated a doctrine of preemption. Certain kinds of international challenges, he said, must be forcibly answered before the evidence of danger is presented to us in the shape of a mushroom cloud. The United States, Bush declared,

can no longer solely rely on a reactive posture as we have in the past. The inability to deter a potential attacker, the immediacy of today’s threats, and the magnitude of the potential harm that could be caused by our adversaries’ choice of weapons, do not permit that option. We cannot let our enemies strike first.