Discrimination Details Matter: Clarifying an Argument About Low-Yield Nuclear Warheads


Perhaps the most controversial element of the 2018 Nuclear Posture Review has been the push to deploy a low-yield submarine-launched ballistic missile (SLBM) warhead. In a recent essay on War on the Rocks, Vipin Narang describes the so-called “discrimination problem”: If an SLBM is launched, he argues, an adversary will not be able to tell that it is carrying a single low-yield warhead. They will therefore have to assume a massive nuclear assault is underway and respond accordingly.

This is a very serious defect if true, as the entire point of a low-yield submarine-launched warhead is to create more limited options for nuclear response. Yet the devil is in the details. There is strong reason to believe the problem of discrimination is both more complex and less dangerous than Narang argues.

The missile in question is the Trident D5, which is used by the both the U.S. Navy and the British Royal Navy. These navies literally share a common pool of missiles, so U.S. and U.K. Tridents are identical including in terms of the signatures an adversary’s early warning system might detect (e.g. the heat signature of the missile’s rocket motor). So any adversary detecting a Trident launch would have no idea whether it was a U.S. or U.K. Trident. If the launch came from the Pacific, the adversary could reason it was a U.S. launch, but this would still be a guess. If the launch was in the Atlantic, it would be a sporting proposition to determine which of the two countries had launched the missile.

Other than a scenario in which the United States was in a nuclear war that the United Kingdom was sitting out — or vice versa — all potential adversaries already have an SLBM discrimination problem. This has been true for decades. While it is conceivable the United States or United Kingdom might be involved in a nuclear war without the other — indeed, this possibility is a major reason the United Kingdom maintains its deterrent — it is a very unlikely scenario in the foreseeable future, particularly with regard to conflict with Russia.

The indistinguishability of U.S. and U.K. Tridents matters because the United Kingdom has had a low-yield SLBM warhead yield for decades — meaning the proposed development of U.S. low-yield warheads doesn’t change much. The late Sir Michael Quinlan, former Permanent Undersecretary of State for Defence and dean of U.K. nuclear policymakers, summed up this reality:

It has long been assumed, and was briefly acknowledged by the government in its December 2006 White Paper on the future of UK nuclear capability, that some of the Trident warheads were of an explosive yield deliberately reduced to considerably below that of the standard warhead … The government has said that in the post-cold war setting it no longer regards terms such as “sub-strategic” as appropriate descriptions of this alternative capability, but it is clear that this is a change of verbal practice, not of substantive policy.

If an adversary detects a Trident launch today it could be a U.S. missile with multiple W88 high-yield warheads; a U.S. missile with multiple W76 warheads; a U.K. missile with multiple Holbrook warheads (roughly equivalent to the W76); or a U.K. missile with a single low-yield version of the Holbrook warhead. The adversary would not know how many warheads were incoming until radars detected reentry vehicle separation and even then it would almost certainly not know whether it was a U.S. or U.K. missile, nor would it know the yield of the warhead(s), as Narang notes.

If the United States were to deploy a low-yield warhead it would, like the United Kingdom, presumably only place a single warhead on a Trident. Adding this option therefore does nothing to affect the discrimination problem in the vast majority of imaginable scenarios. A Trident launch today could be a single low-yield U.K. warhead — the decisions of the 2018 Nuclear Posture Review simply mean that in the future it could be a single low-yield U.S. or U.K. warhead. But in either case an adversary detecting a Trident launch could be facing a single low-yield warhead or multiple higher-yield warheads. If Narang truly believes today’s status quo is a problem, he should be arguing for the United Kingdom to abandon its low-yield warhead rather than criticizing an analogous U.S. deployment.

Arms control also helps with the discrimination problem because it gives adversaries information about how many warheads are deployed on a missile. Narang claims:

Currently, if an adversary were to detect a launch of a Trident missile from an American ballistic missile submarine, there would be no uncertainty about what is coming its way: a strategic nuclear launch of at least about a megaton of yield, perhaps 3.6 megatons.

But he assumes any U.S. Trident will carry eight warheads — either eight W76 (allegedly about 100 kilotons of yield) thus yielding nearly a megaton of yield, or eight W88 (allegedly about 455 kilotons) thus yielding about 3.6 megatons. The New START agreement calls this math into question. Under the treaty, the United States is considered to have deployed 12 Ohio-class submarines with 20 Trident tubes each for a total of 240 deployed Trident missiles. If each had eight warheads mounted the United States would be well over the treaty limit of 1,550 deployed strategic warheads. Because the United States has intercontinental ballistic missiles and heavy bombers that also count toward the limit, the reality is each U.S. deployed Trident will almost surely have five or fewer warheads, not the eight Narang claims.

One can argue that five versus eight warheads is not a meaningful distinction, but this seems to cast aside the logic and utility of arms control, which has for decades made steady reductions in deployed warheads of exactly this sort. Moreover, the three additional warheads could have a combined yield of over 1.2 megatons- well more than 50 times the yield of the bombs dropped on Hiroshima and Nagasaki, which is surely meaningful.

The Russians are aware of this and indeed will have a good sense for how many warheads are on any given Trident missile due to New START verification measures. Amy Woolf, the Congressional Research Service’s expert on strategic forces, describes the process:

The database in New START will list the aggregate number of warheads deployed on all the missiles at a given base, but before beginning a Type One inspection, the team will receive a briefing on the actual number of warheads deployed on each missile at the base. During the inspections, the parties will have the right to designate one ICBM or one SLBM for inspection, and, when inspecting that missile, the parties will be able to count the actual number of reentry vehicles deployed on the missile to confirm that it equals the number provided for that particular missile prior to the inspection … Because these inspections are random, and occur on short notice, they provide the parties with a chance to detect an effort by the other party to deploy a missile with more than its listed number of warheads.

The Russians will thus know that a U.S. Trident launch will be unlikely to have more than five warheads. They would also be able to verify U.S. claims that it has deployed a single warhead on a Trident (though they could not verify yield) through inspections for as long as New START lasts. Russia has a very good sense for the range of warheads it is facing after it detects a single Trident launch (somewhere between 1 and 5) even before its radars identify warhead separation. Indeed, based on New START data, a single Trident launch might seem more, rather than less, likely to be a single-warhead missile. This does not seem conducive to the spasmodic response Narang worries about. Note that by this logic, proponents of a low-yield SLBM warhead should be in favor of extending the New START verification procedures in the future, as they give the Russians greater confidence that a single Trident launch is in fact a limited response.

Narang also worries that if adversaries detect one launch, “doubts about their early warning system could lead them to believe many more are headed their way.” But this would be true of any nuclear use by the United States (or United Kingdom). For example, the United States currently has only a single warhead on each of its Minuteman III intercontinental ballistic missiles, and as noted the Russians can readily verify this. So the Russians should have high confidence if they detect a single Minuteman launch it is only carrying a single warhead. But they might still believe that they failed to detect other Minuteman launches, so many more warheads are on the way. This could lead to an overreaction.

While this could be worrisome, the point is that it is not a worry uniquely caused by the United States deploying low-yield Trident warheads. U.S. nuclear policy has for decades called for “limited nuclear options” using strategic forces — a policy the latest Nuclear Posture Review reiterates. While Narang is skeptical adversaries can distinguish much of anything, this policy has been based on an assessment that Soviet, and now Russian, early warning and attack assessment systems can distinguish between a large attack and a single launch. The United Kingdom obviously has a similar, if differently expressed, policy, which led it to develop a lower-yield warhead in the first place. If Narang and others believe limited use of strategic forces is impossible, that is a different and much larger debate — but one not affected by the existence of a low-yield SLBM warhead.

Finally, context matters. The 2018 Nuclear Posture Review articulates the role of the low-yield warhead largely as part of potential response options to Russian limited nuclear use. While I do not believe the Russians are as eager to use nuclear weapons as the review portrays, they are prepared under some circumstances for limited nuclear use. Logically, if the Russians believe their own limited nuclear use is possible, they have to believe U.S. or U.K. limited use is likewise possible. So, if the Russians launch a limited nuclear attack during an ongoing conflict and subsequently detect a single Trident launch, they will be likely to interpret it as a limited response. This is quite different from a “bolt from the blue” launch of a single Trident outside the context of an ongoing conflict, which neither the United Kingdom nor the United States would ever contemplate.

Nuclear forces and nuclear strategy are complex and the facts surrounding them are often shrouded in classification. Yet enough is known to conclude that if the United States were to deploy a low-yield SLBM warhead, the discrimination problem would be at worst the same as it is today. While there may be other valid critiques of the new Nuclear Posture Review, the discrimination problem is not one of them.


Austin Long is a senior political scientist at the nonprofit, nonpartisan RAND Corporation and the author of multiple studies on nuclear strategy.

Image: U.S. Navy/James Kimber