Strategy is the art of acquiring, deploying, and using force for political purposes. Nuclear strategy involves the design, deployment, and doctrines governing nuclear weapons. Because they are so much more powerful than any armaments previously known, their introduction has required a rethinking of the strategic principles. Deterrence The major political purpose of nuclear weapons, and the centerpiece of U.S. nuclear strategy, is deterrence, or prevention by threat. (State A seeks to prevent state B from attacking, by threatening to respond forcefully to an attack. If B takes the threat seriously and refrains from attacking, A's deterrence policy has succeeded.) Nuclear weapons lend themselves particularly well to deterrence because they can inflict enormous damage on an enemy. Deterrence has become the principal--indeed, some would argue, the only--strategic purpose that nuclear weapons serve. Nuclear weapons policy, to date, has been first of all to deter a war, and only secondarily to fight one. Nuclear deterrence is powerful because the punishment that the deterrer can inflict on the homeland and the society of the aggressor is enormous. Both the United States and the Soviet Union can plausibly threaten to inflict it under any circumstances. This property of their nuclear arsenals is expressed by the term "assured destruction," coined in the early 1960s to describe the standard that the American nuclear arsenal had to meet. It had to have the capacity to destroy between one-fifth and one-third of the Soviet population and between one-half and three-fourths of Soviet industry in a retaliatory strike, even after absorbing an all-out Soviet attack. Both the United States and the \TUSSR\t achieved this capacity for assured destruction in the 1960s. Neither could attack the other without risking an annihilating retaliatory salvo. Theirs became a relationship whose stability was founded on the mutual capacity for assured destruction, the so-called balance of terror. In part a deliberate policy, assured destruction was also a consequence of the weapons that each side had acquired. The easing of cold-war tensions during the later 1980s, the signing (1987) of the Intermediate-Range Nuclear Forces (INF) Treaty, and the continuing U. S.-Soviet negotiations on long-range nuclear missiles seemed, to many, to signal the waning of the era of nuclear threat through nuclear deterrence. Yet the strategy that has developed in conjunction with these weapons will probably continue to operate in some form as long as both sides maintain nuclear arsenals. Nuclear Weapons The vast majority of the world's \Lnuclear weapon\ls are in the possession of either the United States or the Soviet Union and are mainly although not exclusively devoted to the task of deterrence through the threat of retaliation. These weapons may be classified in a number of different ways. They can be divided according to the physics of their explosive capacities: fission, or \Latomic bomb\ls, and fusion, or \Lhydrogen bomb\ls, which are far more powerful. Virtually all existing explosives are based on fusion. Explosives can be distinguished according to their power, measured in tons of \TTNT\t equivalent. Atomic bombs were equivalent to 15,000-20,000 tons of \TTNT\t, or 15-20 kilotons. The power of fusion bombs is measured in millions of tons of \TTNT\t, or megatons. Bombs of up to 60 megatons have been tested. Virtually all those in the inventories of the two great nuclear powers are much smaller, averaging one megaton or less, because more destruction can be obtained from the use of many smaller explosives than from a few enormous ones. Nuclear weapons may also be classified by their range, a classification that is important because it forms the basis for the \Tarms control\t negotiations between the United States and the Soviet Union. Armaments of intercontinental range (intercontinental \Lballistic missile\ls, or ICBMs) have been the subject of the main negotiations, the \TSALT\t and START talks. ICBMs are based within each country or on submarines undersea (SLBMs), and are targeted at the opposing side. Prior to the INF Treaty, intermediate range weapons (IRBMs) were both based in and aimed at targets in Europe. Most battlefield, or tactical, nuclear weapons are also based in Europe. They have the shortest ranges of all, and are the closest nuclear equivalents to the artillery of traditional war because they are deployed chiefly to strike at enemy combat units rather than military and civilian installations far from the front lines. The most important distinctions among nuclear weapons involve the delivery systems that carry the explosives to their targets. Manned aircraft carry free-falling gravity bombs; land- or sea-based ballistic missiles are self-powered and self-guided; the hybrid \Tcruise missile\t, a small, pilotless drone, can be fitted with a nuclear tip and launched from land, ships, and even airplanes. Delivery systems, in turn, may be categorized according to whether they reinforce or undercut deterrence. This depends on whether their characteristics make them better suited to swift, preemptive "first" strikes, which are considered incompatible with deterrence, or to retaliatory "second" strikes, which constitute the essence of deterrence. Convention has it that first-strike weapons are those suitable for attacking enemy weapons, while second-strike armaments are suitable for targeting enemy cities. The first are called "counterforce" weapons; the second, "countervalue" weapons. To aim at weapons is to try to disarm the enemy, to strike with impunity, but armaments aimed at cities are useful only in a retaliatory attack. Hence, these are deterrent armaments. Whether a weapon is better suited to a first or a second strike depends upon its vulnerability to preemptive attack, its speed, and the accuracy with which it can be aimed at its target. Weapons vulnerable to an enemy attack are first-strike weapons, since they would not be available for a retaliatory strike. Invulnerability, therefore, is the essence of deterrence. Submarine-launched missiles are the least vulnerable, because submarines cannot be tracked and destroyed. Missiles deployed on land, even if housed in reinforced concrete underground silos, have become increasingly vulnerable as targeting accuracy has improved. Weapons that can move swiftly from launch to target are more useful for first strikes than slower ones. If a weapon is slow, warning of its approach is available to the other side. Ballistic missiles are swift and thus more suitable for first strikes--and less plainly weapons of deterrence--than are the slower cruise missiles. Finally, weapons that are accurate lend themselves to first-strike counterforce attacks. Greater accuracy is required to knock out hidden, protected missiles than to pulverize cities, which, after all, cannot be hidden, protected, or moved. All these theoretical distinctions do not, however, translate perfectly into policy. Both the United States and the \TUSSR\t have taken steps to improve their capacities for retaliation by reducing the vulnerability of their nuclear weapons. At the same time, both arsenals have become steadily more accurate. And both sides aim at the other's weapons as well as its cities. Yet, deterrence remains in force because each side has so many weapons that, in a preemptive strike, neither could hope to knock out all of the other's armaments. Terrible retaliatory punishment would still await the attacker. Evolution of Nuclear Strategy The first atomic bombs dropped on the cities of Hiroshima and Nagasaki in 1945, were used, like conventional bombs, to crush the morale of the Japanese. In the early postwar years, the U.S. Air Force, whose planes carried the bomb, regarded it merely as the latest and most powerful explosive available. In the decade following the end of the war, nuclear weapons became more central to U.S. foreign policy, and their strategic purposes began to change. The United States undertook to defend Western Europe against Soviet attack. With their large army the Soviets were conceded the advantage in non-nuclear weaponry, but the U.S. nuclear arsenal--the only one until the first Soviet test in 1949, and by far the larger one for a decade to come--was intended to offset this advantage. If the \TUSSR\t attacked Western Europe, the United States would deliver a crushing nuclear blow to the Soviet Union. After the Korean War, the U.S. government pledged to defend a number of Asian countries against communism. Stationing troops in these countries would have been forbiddingly expensive, and it was therefore decided to protect them by threatening a nuclear response to aggression of almost any kind--to deter a broad spectrum of challenges by the threat of "massive retaliation" in response to any of them. This strategy was altered, as the \TUSSR\t built their own arsenal of ICBMs. The U.S. government responded in two ways. First, it sought to protect the U.S. nuclear striking force against a Soviet preemptive attack: it reduced the vulnerability of its weapons, to give the American arsenal the capacity for "assured destruction." Second, it sought to develop forces that could resist Communist aggression of all types without resorting to nuclear war. This was the policy of "flexible response." Nonnuclear assaults on the United States or its allies were to be deterred not only by the threat of nuclear retaliation, but by the capacity for defense at lower levels of violence. Flexible response was the last major change in U.S. strategy, and from the 1960s to the 1980s it remained the official U.S. doctrine. The United States sought to deter a Soviet nuclear attack by the assured capacity for a devastating nuclear response, and to deter lesser forms of provocation with lesser armaments. Three important areas of uncertainty remained, however. First, if deterrence should fail, was it possible to fight and win a nuclear war? Second, did the simple capacity for assured destruction--which a small handful of weapons could provide--make for an adequate deterrent? Third, would new technologies change the basis for nuclear strategy? Dilemmas of Nuclear Strategy In a large-scale nuclear war, one in which the two sides used all, or most, or even a large fraction of their nuclear weapons, each would suffer such catastrophic destruction that neither could regard the outcome as a victory. To provide a victory a nuclear war would therefore have to be severely limited. The prospects for limiting such a war are not at all clear, because the world has no experience of nuclear combat on any scale. Perhaps the United States and the \TUSSR\t would be able to restrain themselves and avoid massive damage--they would certainly have every incentive to do so. But the history of wars between great powers, especially in the 20th century, reveals a willingness to escalate. Nuclear war would certainly create a thoroughly unfamiliar environment, one likely to be more chaotic than any known before. Thus, even if both wished to do so, the great nuclear powers might quickly loose the resources to limit the war's extent. In addition, the two governments are connected to their nuclear arsenals through complex electronic networks of command, control, and communication. These themselves are vulnerable to attack and might not function properly in time of war. (See also \Telectromagnetic pulse\t.) In the 1980s another prospective danger came to light. Scientists hypothesized that nuclear explosions might cause massive damage to the Earth's environment. They could create such intense fires and spew so much material into the air that the sunlight would be blocked out, producing a "nuclear winter" that could, in turn, have fatal consequences for all the planet's inhabitants. It was uncertain how many nuclear explosions would trigger these environmental effects, or whether they would occur at all. The possibility that they might occur has to be counted, however, and is another element of uncertainty affecting the question of whether nuclear "victory" is possible. The policies of the United States and the Soviet Union are ambiguous on the victory issue. Both are well aware of the dangers and uncertainties surrounding nuclear war, and neither has shown any disposition to fight one. Some official U.S. documents--the best known is perhaps a leaked Pentagon paper (1982) detailing a plan to prepare the United States for a lengthy nuclear war with the \TUSSR\t--seem to accept the likelihood of nuclear combat; but the existence of such documents may reflect the need to have some recourse should deterrence fail. In general the Soviets seem to believe that the better prepared they are to wage a nuclear war the less the likelihood that they will have to do so. Overabundance of Nuclear Weapons Because they are so powerful, and can be invulnerable, a relatively few nuclear weapons can give a country the capacity for assured destruction. Both the United States and the \TUSSR\t have many more than the minimum necessary. The United States, especially, has strategic reasons for this surplus. It is committed to the nuclear protection of Western Europe and Japan. Assured destruction is generally reckoned adequate to deter a direct attack on the United States, but not sufficiently credible to deter one against a distant ally. The Soviets, it is feared, might not believe that the United States would risk the annihilation of Washington to protect Bonn. The United States attempts to ensure the "credibility" of its commitment to Europe by "coupling" America's nuclear arsenal to the defense of Europe, and by possessing nuclear firepower over and above the amount required for assured destruction. More generally, the U.S. government tends to believe in the need to match the Soviet Union at every level of force. For example, one rationale for the controversial land-based, highly accurate \TMX missile\t is that it would match similar weapons the Soviets already have. Future of Deterrence The viability of deterrence strategies could be threatened by any one of several improvements in technology. The most conspicuous technical advances in nuclear weaponry in the 1970s, for example, involved the accuracy with which explosives could be aimed. Some weapons on both sides have become increasingly vulnerable to preemptive attack. Those carried by submarines are still protected, but if a way were found to destroy simultaneously all the opposing side's missile-carrying submarines, the country that did so would gain a decisive advantage and could, theoretically, launch a preemptive nuclear strike without fear of retribution. Neither side is close to possessing such an anti-submarine capacity; but other kinds of technological advances, whose effects will be at the very least destabilizing, are at the beginnings of development. The most important potential technological development is the means to defend against nuclear attack (see \Tantiballistic missile\t). Stability presently rests on the absolute supremacy of the offense, the essence of assured destruction. The development of an effective defense would make for a revolution in nuclear strategy. The task of defense against nuclear attack is a difficult, perhaps an impossible one. Each side has thousands of explosives, which can be launched from different directions, at different speeds, and with decoys to confuse a defense. To stop all of them is impossible, and if only one penetrated a defensive system, it could cause catastrophic damage. Nevertheless, the idea of defense is logical and appealing. A world in which safety depended on mutual self-protection is much more attractive than the present world, where it is based on the mutual threat of annihilation. New technologies are constantly invented. Some of them, such as the space-based laser weapons that are being developed under the American \TStrategic Defense Initiative\t (the "Star Wars" program), are classed as defensive weapons; they are designed to destroy offensive nuclear missiles in space. Lesser Nuclear Powers Great Britain, France, and the People's Republic of China possess nuclear weapons. Many smaller powers (Argentina, Brazil, India, Israel) have tested nuclear devices, or possess the technology to make them, and still others have been given nuclear weapons by one of the major powers. The belief that a particular nation might have a nuclear weapon has undoubtedly had some deterrent effect on its adversaries. This is perhaps the ultimate testimony to the strategic impact of nuclear weapons: deterrence is the product of the mere rumor that they exist. Michael Mandelbaum Bibliography: Betts, Richard K., Nuclear Blackmail, Nuclear Balance (1987); Bobbitt, Philip, Democracy and Deterrence: The History and Future of Nuclear Strategy (1988); Bundy, McGeorge, Danger and Survival (1988); Cimbala, Stephen J., Nuclear Endings: Stopping War on Time (1989) and Nuclear Strategizing (1988); Dyson, Freeman, Weapons and Hope (1984); Halperin, Morton H., Nuclear Fallacy: Dispelling the Myth of Nuclear Strategy (1987); Kegley, Charles W., Jr., and Wittkopf, Eugene R., eds., The Nuclear Reader, 3d ed. (1989); Lebow, Ned, Nuclear Crisis Management (1987); Leebaert, Derek, ed., Soviet Military Thinking (1981); Mandelbaum, Michael, The Nuclear Future (1983) and The Fate of Nations: The Search for National Security in the 19th and 20th Centuries (1988); McNamara, Robert, Blundering Into Disaster: Surviving the First Century of the Nuclear Age (1986); Solomon, Fredric, and Marston, Robert Q., eds., The Medical Implications of Nuclear War (1986).