Conventional 'Replacement' of Nuclear Weapons?

This is the written text upon which Dr. Ford based remarks delivered on November 17, 2010, at the conference on “Conventional Deterrence in the Second Nuclear Age,” sponsored by the Carnegie Endowment for International Peace.

Senior Director for WMD and Counterproliferation, National Security Council

One sometimes hears optimistic talk these days about the possibility that strategic-range, precision-guided conventional weapons hold out the prospect of allowing us to reduce our reliance upon nuclear weaponry – potentially completely. I'd like to offer a note of skepticism on this score, though I am certainly not suggesting that "conventional prompt global strike" (CPGS) technologies are not very valuable, or that they cannot at least somewhat reduce our reliance upon nuclear tools. My point is merely that there may be limits to how far such "replacement" arguments can go.

It has now been quite a few years since people first started to become aware that improvements in the accuracy of conventional weaponry held the potential to allow non-nuclear arms to accomplish at least some of the military missions it was long felt could only be undertaken by nuclear weapons. In the mid-1970s, a group of Pentagon strategists known as the Long-Range Research and Development Planning (LRRDP) program began to consider the potential of precision conventional weapons to allow a de-emphasis upon nuclear weaponry for some tasks.

From their work on the potential impact of emerging technologies such as microcomputing, GPS guidance, and autonomous terminal homing, LRRDP experts realized that when it came to destroying a single-point (and surface) target such as a missile silo, a tenfold increase in accuracy did as much good as a thousand-fold increase in explosive yield. The corollary to this, in turn, was that sufficient increases in accuracy could allow enormous reductions in the explosive power of the weapon needed to eliminate such a target – perhaps even to the point that nuclear yield would not be needed at all.

As a result of this realization, LRRDP members including Albert Wohlstetter suggested that new generations of advanced precision-guided conventional armaments might to some degree "potentially trump[] the so-called nuclear revolution" in military affairs.[1] As it turned out, a modern cruise missile with a high-explosive warhead and very good precision guidance – coupled, of course, with top-notch intelligence-derived targeting information, so one knew where to direct it – could destroy a simple point target just as effectively as a nuclear weapon delivered with the much less accurate Polaris submarine-launched ballistic missile system that had come on line as part of America's strategic "Triad" in the 1960s. By the 1980s, when such non-nuclear technologies first began to become available, studies undertaken by the U.S. Strategic Air Command reportedly showed that a good many Soviet targets then in the Single Integrated Operational Plan (SIOP) – that is, the U.S. nuclear warplan – could be engaged by conventional weaponry.

Since these early realizations, the issue of whether – or rather, the degree to which – conventional arms can substitute for nuclear weaponry has formed an important, if little publicized, element in strategic inquiry, at least in the United States. In the 1990s, after conventionally-armed "smart bombs" had enjoyed their dramatic public debut with the release of video footage of precision strikes undertaken against Iraq in 1991, calls began to appear for the United States to take advantage of its possession of such capabilities in order to reduce its reliance upon nuclear weapons.

Perhaps most influentially, one of the architects of U.S. nuclear deterrence during the Cold War, Paul Nitze, argued in 1994 that the United States should rely for its strategic deterrent more upon precision conventional than upon nuclear weapons. In 1998, an article in the Bulletin of the Atomic Scientists, followed up on this concept, noting that such technology "could transform the strategic triad and help pave the way for deep cuts in the current U.S. nuclear arsenal." It even urged U.S. officials to "consider unilateral nuclear force reductions" precisely because America possessed such capable precision-guided non-nuclear armaments.

Under the Clinton Administration, the Defense Department began a major study and planning effort exploring how best to leverage conventional capabilities against challenging deeply-buried and often weapons of mass destruction (WMD)-related targets. To my knowledge, however, the administration of President George W. Bush was the first formally to adopt the idea of using increased reliance upon precision conventional arms as a way to reduce reliance upon nuclear weaponry.

In its 2001 Nuclear Posture Review Report (NPR), the Bush Administration called for the establishment of a "New Triad" that would reconceptualize the "classic" nuclear triad of land-based intercontinental ballistic missiles (ICBMs), submarine-launched ballistic missiles (SLBMs), and strategic bombers. Its "New Triad" would not simply consist of different nuclear delivery systems, but would instead be formed by (a) offensive strike systems of both the nuclear and non-nuclear variety; (b) active and passive defenses; and (c) "[a] revitalized defense infrastructure that will provide new capabilities in a timely fashion to meet emerging threats."

In this vision, strategic deterrence was no longer conceived as deriving solely from nuclear strike capabilities. In fact, as the enumeration of the elements in this "New Triad" made clear, defenses and the strategic "hedge" of a productive defense infrastructure were to be deterrent elements formally coequal with offensive strike capabilities. To the extent that the "New Triad" did rely on offensive strike, moreover, nuclear delivery systems were only part of the equation. According to the Bush NPR, "[t]he addition of non-nuclear strike forces – including conventional strike and information operations – means that the U.S. will be less dependent than it has been in the past on nuclear forces to provide its offensive deterrent capability." Non-nuclear weaponry, therefore, was expected to "both reduce our dependence on nuclear weapons and improve our ability to deter attack in the face of proliferating WMD capabilities."

Now, it is one thing to reduce one's reliance upon nuclear weaponry, and quite another to replace it. The Bush Administration did not promise the latter, but it clearly emphasized the hope that precision conventional weaponry could help America reduce its nuclear arsenal. As I explained U.S. policy to a disarmament conference in 2007, for instance, the Bush Administration sought "better ways to accomplish, without nuclear weapons, strategic deterrent missions that previously could only be achieved with such weapons." Nor was this likely to be irrelevant with regard to some potential future nuclear "zero," either. Such improvements in conventional armaments, I emphasized,

"not only speak to how to make nuclear weapons seem less necessary, but also can help provide an answer to the challenge of how to convince a would-be violator that attempting 'breakout' from a zero-option regime would be very much against its interests. Post-nuclear deterrent capabilities, in other words, could make nuclear weapons seem both less necessary for today's possessors and less attractive for those who might consider them tomorrow."

The theme of using improved conventional weaponry to reduce reliance on nuclear weapons has been embraced by the Obama Administration, which has continued the "Conventional Prompt Global Strike" (CPGS) program begun by its predecessor. Indeed, President Obama himself has declared that this work is part of a broader effort "to move towards less emphasis on nuclear weapons," because such non-nuclear improvements can help ensure "that our conventional weapons capability is an effective deterrent in all but the most extreme circumstances." (For an analysis and critique of Obama CPGS thinking, however, see my earlier New Paradigms Forum essay.)

(Fascinatingly, despite Moscow's incessant complaints about U.S. CPGS planning, Russia also seems interested in developing such capabilities. As pointed out in a recent analysis by the Center for Nonproliferation Studies, the Kremlin's newly-released strategic doctrine contains a sharply new element: "it assigns high-precision (apparently, conventional) weapons to the mission of strategic deterrence. This clearly indicates that Russia plans to follow the same trajectory as the United States and equip a growing share of its strategic delivery vehicles with conventional warheads." With even China reportedly now developing a conventionally-armed ballistic missile capable of chasing down an aircraft carrier from nearly a thousand miles away, there thus seems to be an increasing interest among the major powers in fast-moving long-range conventional capabilities.)

The issue of nuclear "replacement," however, forms an important conceptual threshold, for it may be that CPGS-type capabilities are attractive to such power principally as a supplement to nuclear capabilities, as a way merely to reduce exclusive reliance upon nuclear explosives for some potential military tasks. To hypothesize CPGS-type capabilities as a tool to facilitate movement along a road toward some future end-state of nuclear weapons abolition is to take the idea much further, and perhaps too far.

So far, official U.S. pronouncements – even my own in 2007, which at least raised the question of CPGS' utility in a world of "zero" – have been somewhat cagey. This is appropriate, for the field of long-range conventional strategic strike is in technological terms a young and still developing one with ambiguous implications, though the speed at which such missions can increasingly be undertaken may indeed offer a real prospect of replacing more once nuclear-exclusive missions than could merely the accurate but slow-flying conventionally-armed cruise missiles that began to be developed in the 1970s. (Today, programs are underway to explore the placement of conventional warheads on ballistic missiles with intercontinental range or on exotic new hypersonic cruise platforms, both of which could reach targets essentially anywhere in mere minutes.) The question of taking conventional systems to any kind of asymptote in "replacing" nuclear weaponry is certainly one worth exploring.

In some sense conventional weapons – quite apart from CPGS – have already shown themselves capable of "replacing" nuclear weapons for many very important missions. During the Cold War, after all, we relied heavily on nuclear weaponry as a means of forestalling an invasion of Western Europe by numerically superior Warsaw Pact forces. As recognized in the 2009 report of the Strategic Posture Review Commission, however, such conventional-force deterrence missions are no longer salient for U.S. planners: we have conventional military forces powerful enough that we "no longer need to rely on nuclear weapons to deter the threat of a major conventional attack."[5] But the more analytically interesting question for present purposes is whether it might be possible to rely upon conventional forces to replace nuclear weapons in deterring the use or threat of use of nuclear weapons.

In this regard, however, there is some reason to be skeptical. To move beyond "reducing reliance" to the much more demanding challenge of in some sense "replacing" nuclear deterrence would seem to require demonstrating that conventional tools can suffice for essentially any nuclear mission we might feel some need to accomplish in wartime. I am not yet persuaded that this is possible.

To begin with, there is one potential nuclear mission that certainly cannot be accomplished by CPGS-style conventional tools. I refer to what nuclear strategists call "countervalue" targeting, in which the objective is simply the most rapid possible erasure of large urban populations. As became clear during the Second World War, conventional weaponry can indeed obliterate entire cities, but the instances in which this occurred – e.g., the firebombings of Dresden, Tokyo, and some other Japanese cities – involved enormous bombing campaigns in which hundreds or even thousands of heavy aircraft took part. Even if one wished to repeat such horrors today, sufficiently numerous forces are no longer available. Nuclear weapons clearly offer a unique capability to slaughter civilian populations.

This objection, however, might seem fairly easily met, for "countervalue" targeting is today long out of fashion in Western military and strategic policy circles. No one denies that targeting so-called "counterforce" targets – e.g., enemy leadership, command-and-control, and military facilities – can produce enormous "collateral" civilian casualties to the extent that such targets are located in urban areas. The deliberate targeting of non-combatant civilians, however, is now regarded as both immoral and illegal. For this reason, it is easy to argue that the inability of even the most modern conventional weaponry to accomplish the barbarities of countervalue targeting is no obstacle to nuclear "replacement." Since we neither do nor should wish to target non-combatant civilians anyway, the argument would presumably go, "losing" such countervalue capabilities by a transition to a solely CPGS-type deterrent would really be no loss.

But this riposte is a bit too easy, especially – and here's the irony – for proponents of dramatic nuclear weapons reductions. Weapons cuts of the sort long propounded by the disarmament community, after all, tend naturally to push nuclear targeteers away from "counterforce" and back into "countervalue" target planning. A very small nuclear arsenal can essentially only target cities as cities, for it lacks the numbers of deliverable warheads to expend on alternative targets such as missile silos and hardened command bunkers. (To be sure, if one's potential adversary has reduced his armaments too, the number of targetable missiles he has might remain commensurate to one's own force posture, but as numbers fall the relative proportion of non-weapons leadership, command-and-control, and other counterforce targets will increase. Deeply buried or otherwise hardened targets, moreover, may demand the assignment of more than one incoming weapon in order to ensure destruction.)

Counterforce targeting is comparatively expensive in terms of the number of attacking weapons that are needed, and "minimum deterrence" at low force levels is almost inescapably countervalue deterrence. This means that the ability of CPGS-type systems – or any other conventional tools short of thousand-sortie carpet bombing – to "replace" nuclear missions may be in some sense especially constrained at small stockpile numbers, because non-nuclear weapons simply cannot provide the countervalue impact that is inherently likely to be demanded of such limited nuclear arsenals. Even if precision conventional weapons could replace nuclear weapons for any imaginable counterforce mission, they cannot offer any significant strategic countervalue utility and thus would be hard pressed to "replace" a very small nuclear arsenal of the "minimum deterrence" variety urged upon us by disarmament advocates as the next step toward "zero."

In this sense, one might even wonder whether a large force of some kind of hyper-capable CPGS assets might more easily "replace" a sizable, counterforce-targeted nuclear arsenal than replace a small, disarmament-friendly "minimalist" countervalue one, for the missions contemplated for these hypothetical nuclear stockpiles would be quite different. Could one thus imagine transitioning from nuclear armament to a CPGS-based conventional deterrent without first moving to a small nuclear arsenal, and while retaining counterforce targeting throughout the shift?

Maybe, but don't hold your breath, for there is reason to doubt that CPGS really can replace nuclear weapons for all counterforce missions, particularly with respect to prosecuting deeply-buried targets. These targets present a formidable problem.

According to disarmament activist and nuclear command-and-control scholar Bruce Blair, Pentagon planners some years ago reviewed the vast corpus of U.S. nuclear testing data and used modern modeling techniques to reassess the vulnerability of underground structures to nuclear explosions. This new study concluded that prior generations of planners had significantly overestimated the ability of nuclear weaponry to hold such targets at risk.

The problem related to the depth of the craters likely to be produced by nuclear explosions at or above the surface of the earth. In short, these holes weren't likely to be nearly as big as had once been anticipated. (It had previously been estimated, for example, that a one-megaton nuclear weapon – yielding the explosive power approximately equal to that of a million metric tons of TNT – could scoop a crater 485 feet wide out of dry, soft rock, giving the explosion a "lethal depth" of 728 feet for structural damage to hardened underground facilities. Under the revised formulae, however, these figures dropped dramatically, to 180 and 270 feet, respectively.) As a result, the weapons in the U.S. arsenal in 1990 were "no longer credited with the potential to deliver a lethal blow to the deepest of the Soviet command posts."[6] This caused considerable complications for U.S. counterforce targeting, requiring the re-allocation of additional weapons to the deepest targets – in order to provide multiple-explosion lethality – and spurring interest in nuclear earth-penetrator concepts pursuant to which a single weapon might in the future be able more successfully to "couple" the nuclear explosive shock to the rock strata above and surrounding such facilities.

Even if such doubts had not been cast upon the ability even of nuclear explosives to kill hardened, deeply-buried targets (HDBTs) without the delivery of successive multiple warheads, it is doubtful that any sort of conventional weaponry could be relied upon to attack the deepest of counterforce targets. Excellent accuracy can let a high-explosive warhead kill an ordinary point target as well as can the much less accurate delivery of a huge nuclear warhead, but conventional weapons have diminishing returns against HDBTs.

Developers of anti-armor munitions have devised clever "thread-the-needle" ways to attack the thick armor sheaths protecting tanks and bunkers by putting more than one warhead on the same missile, depositing the second one into the small hole made microseconds earlier by the first explosion. This is not unrelated to the idea of using multiple nuclear warheads to dig successive craters deep enough to destroy a buried target, but such techniques surely have sharp limits when trying to place a conventional warhead hundreds of feet underground through rock. If you lack the ability to transfer really huge, nuclear-scale energies to the rock strata that surround a buried facility – thus creating a shock wave capable of killing a facility buried significantly deeper than your warhead can burrow – you have much less chance of doing what you need to do.

Nuclear earth penetrator concepts, it should be stressed, do not aspire to drill their "physics package" down to such depths: they seek merely to get it deep enough that its detonation can transfer energies sufficient to crush facilities still far below. (It is this "shock coupling" that provides the attraction of a nuclear earth-penetration design. As far as I know, it has nothing whatsoever to do with "minimizing" radioactive fallout.) Because conventional explosives pack so much less energy yield per unit of volume, they would have to burrow vastly closer to a deep target before lethality could be ensured. Work is certainly ongoing on just how deep underground one can drive a conventional warhead, but the main problem is one of basic materials physics, and we should not expect miracles. In any event, one can unquestionably kill something far deeper – at any given depth of penetration – with a nuclear explosion than with a merely conventional one based upon the same penetrator volume.

Current concept planning in the United States tends to think in terms of achieving a "functional kill" with conventional weapons by doing such things as blocking access tunnels, rather than by relying upon any ability directly to inflict direct damage on a HDBT. This is about as promising as non-nuclear approaches seem likely to get, but it too has its limits. (Facilities are likely to have multiple entry/egress passages, and tunneling one's way back out is not necessarily very demanding even if the attacker has perfect intelligence about all such access points. Such a "functional kill" would also have to be maintained over time with repeated strikes, making it of limited utility unless as part of a broader campaign – and for only so long as fighting continues and sufficient munitions and attack opportunities remain available.) HDBTs, as we have seen, stress even modern nuclear targeteers, and it is very hard to see how CPGS could entirely close this gap.

This is no doubt one reason why HDBTs have been proliferating in the modern world, particularly for regimes eager to protect their own WMD programs against conventional attack by precision conventional weaponry. (The North Koreans are famously industrious HDBT "diggers," for example, while Iran took great pains to put its Natanz uranium enrichment facility beneath thick layers of concrete and earth, also constructing its Fordow uranium facility in a tunnel drilled deep into a mountainside near Qom. Libya's one-time chemical weapons facility at Tarhuna also enjoyed the protection of a huge rock overburden. Suitable tunneling equipment and expertise is increasingly available around the world.) In light of the spread of these targets, in fact, U.S. planners under the Clinton and Bush Administrations, at least, conceived of nuclear weapons as having at least potential counter-WMD missions against HDBTs that might otherwise be otherwise immune to attack.

Accordingly, the proliferation of HDBTs has emerged as a factor complicating disarmament aspirations. In part this is because it is not clear that CPGS-type tools can threaten well-constructed deep targets, with the result that any would-be "post-nuclear" strike planner must consider whether he can afford simply to abandon HDBT targeting, which complicates the achievement of any potential nuclear "zero." In part, it is also because targeting the toughest HDBTs with nuclear weapons as part of a counterforce strategy may require the assignment of multiple warheads – making it more difficult to reduce nuclear stockpiles down towards "zero" in the first place. (The availability of nuclear earth penetrators would for this reason allow faster reductions in aggregate numbers, but the reflexive politics of "no new nuclear weapons" makes this sort of arsenal reduction unlikely, at least for the United States. Other powers may be less squeamish.)

All of this suggests that while advanced precision conventional arms may help make possible some further lessening of our current reliance upon nuclear weapons, one should not expect some kind of CPGS-type capability to allow the "replacement" of nuclear deterrence, either for countervalue or for counterforce purposes. If one were willing simply to forego targeting what would presumably be the most important HDBTs, the case against nuclear "replacement" would be less daunting, but there is as yet little sign that this would be acceptable to planners who may today still be able to use nuclear weapons to hold many or most such targets at risk as part of their deterrent strategy.

Make no mistake: precisely because CPGS can at least reduce our reliance upon nuclear weaponry, and because it stands to provide considerable utility in attacking time-urgent ordinary targets thousands of miles away – targets such as the al-Qa'ida leadership conclave in Afghanistan that slower-moving conventionally-armed cruise missiles failed to hit quickly enough back in 1998 – CPGS is nonetheless likely to be a valuable addition to the American toolkit. One should not mistake this supplementary tool, however, for a real nuclear "replacement."