The Soviet Union and the United States are rival superpowers not simply because of their wealth, numbers, size, geographical position, social cohesion, strong government, but because they have translated these potentialities into overwhelmingly strong military forces which, measured on any historic or current standard, are comparable only with each other. For nearly fifteen years, the central strength of these forces has been their respective long-range strategic striking arms, each designed to be capable of a large-scale attack with nuclear weapons on the home territory of the other. Over the past decade, more or less, each nation has become increasingly aware that the chief utility of his strategic force was to prevent his adversary from using his own. This result was achieved primarily by offering the adversary the prospect that any attack by his strategic forces would be met by a counterblow so devastating as to convert a decision to attack into a suicide pact. So the strategic equilibrium commonly termed "mutual deterrence" was recognized.

For our (or the Soviets') strategic forces to provide effective deterrence, they must be in such numbers, of such nature and so deployed as to be capable of delivering the required counterattack after the other side has struck; thus effective deterrence is measured by the usable strength of the survivable second-strike force. We were perhaps earlier than the Soviets in recognizing this, but we were far from perceiving it from the first. Once we recognized the need, we sought survivable forces in different ways as technological possibilities changed over the period. Increase in the size of the force, geographical dispersal to increase the number of targets presented by a given force, active defense, hardening to survive attack, warning and movement capability to take advantage of warning, all played a part in the quest for a secure second-strike force. If surviving forces are to be usable, equally essential requirements are imposed for ensuring the survival of a complex network of reporting and communication facilities, command organization and commanders, all of which occasion their own technical and organizational problems. With missiles having displaced aircraft as the most important component of the second-strike force for both sides, hardening to ensure survival through an attack, combined with concealment and mobility for the sea-based portion of our force, provide our main means of ensuring survivability. The Soviets, with-at this moment- smaller and less effective seaborne forces, rely more heavily on hardening.

Strong and survivable long-range striking forces provide each superpower with something more in relation to the other than deterrence against direct nuclear attack, though the precise specification of the extra effect is difficult. First, they provide a substantial incentive for each nation to refrain from initiating any military action against the other, lest the conditions under which rational calculation can be expected to dominate decision and action disappear in one or both. These incentives become stronger the larger the forces and interests involved, thus leading to a kind of built-in brake on the growth of military incidents in situations where the military forces of the superpowers face each other directly, or could readily do so in their worldwide movements. By extension, the same incentives operate with respect to political confrontations that might in turn lead to military action, but more weakly the more remote the military steps appear to be in the chain of potential actions and reactions. Together these effects add up to a kind of indirect or second-order deterrence, which could tend to stabilize the behavior of the two superpowers in relation to each other over a wide range of actions, and deter unilateral attempts by either to change the status quo forcibly or suddenly. This might be called the self-deterrent effect of strategic nuclear forces.

However, the history of the last two decades makes the strength, steadiness and symmetry with which these incentives have operated questionable, and emphasizes their relation to broader military and political contexts. In the earlier part of the period, the Soviets seem to have acted at a higher margin of risk than the United States; more recently, the reverse appears to be true. These changes are not the simple consequence of shifts in the balance of strategic forces; on the contrary, any shift has probably moved steadily against the United States over most of this period. This is not the place to discuss the reasons for these apparent shifts in behavior, but they do illustrate the problems of delimiting the reach of the broader deterrent effects of nuclear striking forces.

In analyzing the concept of effective deterrence and trying to understand on what relation of forces it depends, it is conventional and useful to examine it in the context of a spectrum of possible strategic purposes and the striking forces appropriate to them, that stretches from what might be termed a credible first strike at one end to a minimum deterrent at the other. A first-strike force would be one whose size, reliability, accuracy, control arrangements, etc., were such, in relation to the adversary's forces, as to make possible an attack that would, with a high degree of assurance, destroy essentially all of the adversary's force and still leave the attacker a substantial unspent reserve force. In this context, the sense of "essentially all" of the adversary's force is that whatever residual might escape destruction could not inflict major damage on the attacker or prevent his reserve force from being used to a very substantial extent. For a first strike to be further characterized as "credible," the relation of forces described above would have to be clearly perceived by both adversaries, and the "high degree" of assurance involved might have to be set at 99 percent or more. In such circumstances, it is just conceivable that the superior adversary could use this power for what has been termed "compellance," as opposed to deterrence: the threat of a strike used as a means of compelling specified behavior by the adversary. Just after the Second World War, when the United States still had a monopoly on nuclear weapons and expected to maintain it, there were some who argued in favor of policies based on "compellance." Whatever the wisdom of the policies, the forces then at our command were never such as to provide that power in relation to the Soviets; it is doubtful if there ever was a moment when they were; and it is as certain as any judgment in these matters can be that we cannot now or in the foreseeable future achieve it.

At the other end of the spectrum, a minimum deterrent force would be one which would provide high assurance of the survival, for use in a second strike, of an effective, usable force large enough to inflict unacceptable damage on the adversary, defined in terms of some absolute level of expected casualties, urban and industrial destruction, etc.

Since 1961, under the guidance of Secretary McNamara, our strategic forces have been programed in terms of deterrence-plus. We have never sought a first strike-capacity, and indeed from his first budget message, the Secretary denied both the possibility and desirability of attaining one: secure Soviet striking capacity was and would be maintained at a high enough level to make a U.S. first strike irrational. But, in the first two full budgets of the Kennedy Administration-which laid down guidelines governing the size of the strategic striking forces that are still in effect today-the programed missile and long-range bomber forces were larger in relation to projected Soviet forces than would have been required for minimum deterrence alone, even allowing for a generous margin of uncertainty on the growth of Soviet forces, their effectiveness and the post-attack performance of our own programed forces. The margin over deterrence was justified in terms of the idea of "damage limitation," should deterrence fail-a contingency that could not be ignored. Should sufficient warning of preparations for a Soviet strike or actual launching of one be available, U.S. missiles could be launched against Soviet missile sites and airfields, thus limiting to an extent depending on warning time the damage that the Soviet strike would inflict.

A large enough effort at "damage limitation" of course shades off into a first-strike posture; a small enough one becomes indistinguishable from the safety margin for deterrence. After the third McNamara budget, the emphasis on damage limitation vanished, to be replaced by deterrence, under the terminology of "assured destruction," referring to the capacity of our strategic forces to survive a Soviet attack and achieve with high probability a level of death and destruction in the Soviet Union that would rule out nuclear war as a rational policy for them.

The decisions of 1961 and 1962 called for the build-up by 1965 of a U. S. strategic force of nearly 1,800 missiles capable of reaching Soviet targets; somewhat more than a third were to be submarine-launched. In addition, some 600 long-range bombers would be maintained. This was projected against an expected Soviet force of fewer than a third as many missiles and a quarter as many bombers capable of reaching the United States. Further, the Soviets were expected to possess an equal number of shorter-range missiles and a much larger number of medium bombers that could be used against European targets. These decisions were taken by a new administration in the aftermath of the missile-gap argument that had been a prominent feature of the election campaign; it was a time, too, when American intelligence estimates of Soviet programs for their strategic forces were undergoing sharp revisions, to the accompaniment of the intra- governmental controversy that such processes always entail. Whether, in another atmosphere, such a program might have been viewed as unnecessarily large for deterrence alone, and whether the articulation of an additional objective of damage limitation would have been avoided from the start is, of course, unknowable. Unknown, and perhaps equally unknowable at least for some time to come, is whether the Soviets' original force goals in 1961- 1962 were as modest as our estimates of them at the time-or even more so- and whether their rapid recent build-up, discussed immediately below, was a response to the tremendous acceleration in the growth of our long-range striking forces that the Kennedy Administration brought about. In any event, until 1965 or even into 1966 it was possible for the Administration to deny any wider aim for its strategic posture than deterrence, to argue the futility of seeking to achieve a first-strike force, and yet to avoid the sharp edge of the question of whether we were maintaining "strategic superiority" over the Soviets.

Recent changes in Soviet deployments have given a new bite to this question; corresponding and anticipatory changes in our own have raised even broader questions of how stable our deterrent posture will be in the years ahead.

II

The last two years have shown significant changes in the Soviet strategic forces. The number of their intercontinental and submarine-launched ballistic missiles has grown rapidly. As of late 1967-as estimated by Secretary McNamara in his 1968 budget presentation-the number of land-based missiles had grown to 750, or nearly half our total, and most of the growth had taken place in the previous year. This indicates that the number might well continue to grow rapidly. The total number of Soviet missiles targetable against both the United States and NATO countries is already nearly equal to the total number of U.S. missiles that can reach Soviet targets. Further, there are some suggestions that the Soviets are currently building up their missile submarine fleet both qualitatively and quantitatively, so as to achieve-like the United States-a substantial force protected from a first strike by concealment and mobility.

In addition to these changes in offensive forces, the Soviets have deployed an anti-ballistic missile defense system around Moscow. They may be extending this deployment further, but if so, they are doing it slowly rather than rapidly.

So far, we have not responded to these developments by planning an increase in the number of our missile launchers. Rather, we have concentrated on programs for upgrading our present forces by replacing existing missiles with new ones designed to use present launching platforms. The new missiles will be superior to the old in three respects. First, they will be significantly more accurate, which means that a smaller warhead can be used to achieve a particular level of destruction against a specified target. Second, they will contain a variety of decoys and other penetration aids which will make more difficult the defensive task of an ABM system. Finally, and most significant, they will ultimately contain several independently aimed warheads within a single missile (MIRVs or multiple independent reentry vehicles, to the trade). These, in turn, again make the task of the defensive ABM more difficult. In addition, they raise a new, and as we shall see, somewhat frightening possibility of multiplying greatly the number of warheads which one or the other side can launch, without changing the number of visible missile launchers.

In addition, of course, we have made the decision to deploy a "thin" ABM system, emphasizing primarily area defense against light attacks, a so- called anti-Chinese defense.

In themselves, none of these changes will significantly alter the strategic balance from one of mutual deterrence, if they do not go beyond their present limits. Neither the Soviet ABM nor the growth of Soviet missile forces reduces in an important way our capability to inflict unacceptable damage on the Soviet Union after a Soviet first strike. In his most recent budget presentation, Secretary McNamara estimated that, on the basis of projected force levels in the 1970s, a United States retaliatory attack after a Soviet strike against our strategic forces would result in over 100 million Soviet dead. Similarly, Soviet responses to our proposed ABM deployment, if they were like our own responses to theirs, would be readily within their capabilities and would leave them able to inflict over 100 million fatalities on us in a second strike. Were we to deploy much stronger and more costly defenses, it would remain within the Soviet capability to counter them by corresponding increases in the size of their offensive deployments-which could probably be made at significantly less cost than that of our increased defenses. The Russians would thus retain their capability to inflict high levels of fatalities on us in a second strike.

To be sure, the Administration asserts that our ABM system is being deployed to counter the possibility of a light Chinese attack in the coming years. The Chinese are testing thermonuclear devices and medium-range missiles; within the next decade they could achieve a modest ICBM force. In the same budget testimony, Secretary McNamara argued that the proposed ABM system might hold fatalities from potential Chinese attacks of varying plausible sizes in the next five to eight years to below a million; in the absence of such a system the figure could range from seven to fifteen million. But why deterrence, on which we have relied successfully for many years to restrain the powerful forces of the Soviets, is not expected to work against the small forces of the poorer and weaker Chinese is totally unclear. For some time into the future, at least as far as our present projections and calculations go, we shall have a credible first-strike capacity against the Chinese. In such circumstances, it is difficult to understand on what calculation the Chinese would throw away the only effective restraint on our nuclear force, the strong moral and political inhibitions we have against striking first, by launching an attack.

Only the proposition that there is a special danger of aggressive irrational behavior by some future Chinese Government would make sense of the official argument. Yet there is nothing in the recent record of the Chinese Government in its international relations that provides the basis for such an expectation. Accordingly, it is hard to take the Administration's rationale for deploying a thin ABM defense at face value, especially in the light of its own evaluation of the utility of not only the proposed defense system, but more elaborate, extensive and powerful ones. The decision is much more easily understood as the result of long- standing pressures within the military establishment and the Congress for an antimissile defense. When these were reinforced in recent years by the evidence of initial Soviet deployments, the demand for a "response" finally became irresistible.

While the recent and projected changes in Soviet and American strategic forces have not altered the fundamental strategic situation from one of mutual deterrence, they may have set in motion forces which will undermine the stability of the relation in coming years. Within the Congress, pressures are already beginning to mount for action to offset the large increase in numbers of Soviet missiles, so as to maintain a margin of "strategic superiority," rather than accepting "parity." As the planned deployment of the ABM system goes forward, Congressional and public pressures to upgrade it can be expected to rise; this would be done by adding local defenses of missiles and cities to the present area-defense system. The next Administration will then face the task of making the public case for the explicit acceptance of strategic "parity" with the Soviets, or in the alternative, giving in to these pressures and beginning a new set of developments in our strategic forces, with consequences that are unpleasant to contemplate.

The core of the case for accepting "parity" has already been put, but it bears repetition, and a little elaboration. In essence, we cannot expect with any confidence to do more than achieve a secure second-strike capacity, no matter how hard we try. This capacity is not usefully measured by counting warheads or megatons or, above some level, expected casualties. Whether the result comes about with twice as many American as Soviet delivery vehicles-as has been the case in the past-or with roughly equal numbers, or even with an adverse ratio, does not change its basic nature. Further, any significant change in deployments by either major adversary requires a long period of time, and announces itself, either explicitly or through intelligence means, in its early stages. The other, therefore, has notice and time within which to respond. The present level of research, development and production capacity for weapons of both superpowers is such that each has the power to respond to a change in the deployments of the other in a way that leaves it "satisfied" with its new position in relation to the adversary. Each, accordingly, must anticipate such a response. And so the arms race would go on. The expected result of the process can be no more than a new balance at higher force levels, larger expenditures and, most likely, even more unthinkably high levels of destruction in the event that the forces were ever used.

Moreover, there is another and even more troubling consequence of following the competitive path: the stability of mutual deterrence becomes far less certain in a rapidly changing situation. Deterrence is at bottom a political and psychological concept. It rests on the perception and interpretation of the military situation by political decision-makers; and it is as open to influence by changes in their operating environment or the attitudes they bring to their perceptions as by changes in the hard technical facts. This inevitably marks it with a certain elusive-ness. How great a capacity to wreak death and destruction on an adversary is enough? Can it be measured in absolute terms in millions of dead and acres of destruction, or only in the fractions of his population, industry and urban area? If one rival's destructive capacity grows while the other's remains constant at a high level, does this reduce the effectiveness of the latter's deterrent? Questions such as these clearly have no unique, well- defined answers that hold for all decision-makers in all circumstances. What is clear is that constant or slowly changing force structures, whose technical performance characteristics are reasonably well understood- subject, of course, to the important fundamental limitation that they have not been and never can be tested in a realistic way-provide a much more stable basis for mutual reliance on and acceptance of deterrence than a rapidly moving process of qualitative and quantitative competition.

The behavior of a whole defensive system in action is difficult to predict, since it depends heavily on specific details of design of both offensive and defensive weapons which are not likely to be well known to both adversaries. The effects of the interaction of two strikes, in which offensive and defensive forces on both sides are involved, multiply the uncertainties. Thus the "adequacy" of any particular second-strike force becomes harder to assure with confidence, and the impulse to compensate for uncertainty by building larger forces will make itself felt on both sides.

Further, current technical developments in weaponry present the possibility of introducing significantly new elements of uncertainty into the situation, which in themselves, even in the absence of further continuing change, would diminish the stability of deterrence. Anti-ballistic missile defenses constitute one example; another is the multiple, independently aimable warheads carried by a single missile. At present, each adversary has a reasonably clear idea of the other's deployments, with enough detail to permit a confident estimate of the balance of forces. Once MIRVs become widespread, it becomes much more difficult for each side to know how many warheads, as opposed to launchers, the other has. How great the uncertainty will be depends, of course, on the specific technical possibilities: two warheads per missile would create one situation; ten, quite another. In the absence of submarine-launched or equally mobile land-based missiles, the latter situation could give rise to the ugly possibility of each side's possessing a first-strike capability against the other, provided the accuracy of the independently aimed warheads permitted successful attack on hard targets.

III

These frightening possibilities still lie in the future. For the present, and the next year or two ahead, nothing that is currently happening or in prospect justifies anxiety for the continued effectiveness of the U.S. second-strike capability, or its continued power to perform its primary function of deterring the Soviets from using their nuclear forces against us. None of the evidence on the Soviet build-up points beyond an effort to move close to a crude equality with us in numbers of offensive missiles. There is no evidence of a wide program of ABM deployment; indeed, nothing beyond the Moscow system seems to be happening. This is certainly no more imposing than our own proposed defensive system, which we believe to be too weak to affect our calculation of Soviet deterrent capabilities.

The rapidity of the recent build-up in Soviet forces tempts some to project that build-up into the future, and see it as a race for "strategic superiority." The absence of official announcements of force goals by the Soviets-such as the Secretary of Defense makes in his annual budget presentations-reinforces the temptation; and the demonstration of the illusory nature of such a goal seems an insufficient response. Yet our past experience with the projected "bomber gap" of the early fifties and the "missile gap" of 1959-61 shows the problems of such interpretations. In both these cases we clearly over-reacted. In the first, the result was our concentration on the creation of a very large, expensive and vulnerable bomber force, while the Soviets moved on to missiles. In the second, the scale of our reaction has probably been the major factor in stimulating the current Soviet build-up. If our aim remains that of maintaining deterrence, we can clearly afford to wait for the event, rather than begin now to respond to our projections of the future.

Can our aim be more? Should we seek to exploit the opportunities of our new technology as rapidly as possible to reach beyond deterrence ourselves? There are strong forces in the military and the Congress always ready to urge this path. They are undoubtedly supported, and their voices amplified to some extent, by the handful of large contractors who benefit from an increase in expenditures on strategic forces. What is many times more important, they reflect a deeply held and widely shared popular sentiment that our wealth, our power and our virtue entitle us to be "first," and any claim to equality by the morally and economically inferior Soviet Union is at best presumptuous, at worst dangerous.

The facts of weapons technology have long made the pursuit of "virtue" along this path vain and wasteful; now they make it positively dangerous. We are in a position in relation to our superpower adversary in which, for the present and near future, the balance of forces is such as to make possible new and far-reaching arms-control agreements. With not too widely disparate levels of offensive forces, and some deployment of defensive forces on both sides, the possibility of an agreed "freeze" in further deployments of strategic weapons looks more favorable than it has for some time. Such an agreement would clearly present a host of difficult problems, centering around how much control of technical improvement in existing vehicles and warheads was sought, and by what means. No risk-free solution of these difficulties is likely to be found, even in concept, much less in terms of negotiable arrangements between the two countries. Yet, the alternative risks arising from the uncontrolled forward thrust of technical change in weaponry are much greater. The more successful the development of MIRVs in terms of both the number of reëntry vehicles carried by a single warhead and the accuracy with which they can be directed, the more difficult it becomes to be confident about the security of a deterrent force. Wider deployment of ABMs compounds the difficulty by adding to the uncertainties of both sides as to the effectiveness of their own and their rivals' forces. Under such circumstances, arms-control agreements in turn may become much more difficult to reach, since an atmosphere of mutual distrust and fear will hardly promote the success of what will at best be difficult negotiations.

All signs point to the coming of a time when the stability of mutual deterrence can no longer rest reliably on mutual watchfulness and forbearance, without explicit arms-control agreements over strategic weapons deployments. We should do nothing to hasten its arrival, and everything to take advantage of whatever respite we have to move forward to such agreements.

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