Trump’s call for more nuclear weapons, reported NBC News, “boggle[d] nuclear experts“:

While President Barack Obama has proposed a multibillion-dollar plan to modernize the aging U.S. nuclear triad, no mainstream voices are arguing to increase the numbers of nuclear weapons beyond the 4,500 active warheads the U.S. currently possesses, several experts told NBC News.

Indeed, over at the New York Times, Max Fisher attempted to interpret the Tweet. Was Trump referring to modernization—a policy supported, at least to some degree, by most but opposed by some on the left—or an expanded nuclear arsenal? In a subsequent piece, Fisher laid out the consensus case against the latter:

Mr. Reagan principally turned against the arms race because of its dangers, but others came to oppose it for the simple reason that, after decades and billions or perhaps trillions of dollars, it had failed to accomplish victory. “Building nukes to get others to stop historically has had the same effect as telling everyone in an email storm to cease using ‘Reply All,’ ” Joshua H. Pollack, an expert at the James Martin Center for Nonproliferation Studies, joked on Twitter. Mr. Pollack added, “There is no last, winning move when it comes to arms racing.” The first response came from Cheryl Rofer, a retired nuclear scientist at the Los Alamos National Research Laboratory: “But there is a last move.”

But, in fact, there is a small minority of scholars and analysts who do support building more nuclear weapons. One is my friend and colleague, Matt Kroenig. Here, he debates with Joe Cirincione on PBS Newshour. And here is his recent piece in Politico. What Matt thinks matters, both because he’s one of the strongest voices advocating for more nukes, and because, despite being vocally #NeverTrump, there’s a nonzero chance he’ll land in a policy position in the administration.

It should come as little surprise that I think he’s wrong.

First, as Joe points out, when Matt argues for the existance of a “gap” in US and Russian capability, he’s pointing to nuclear warheads rather than delivery capability—that is, the ability to strike your opponent with nuclear weapons. The gap is likely smaller when you include non-deployed nuclear weapons. None of this includes UK or French nuclear capabilities.

The differences at stake here simply aren’t, in my view, enough to worry about. But, more important, we still lack the kind of compelling evidence necessary to guide policy. Matt does have a few academic articles showing that relative number of nuclear weapons can affect crisis outcomes, but the handful of recent studies on how nuclear advantage shapes state interactions points in different directions. You can read a debate on this subject that we hosted at the Duck of Minerva, as well as Erik Voeten’s thoughts on the findings.

Second, as you’ve already guessed, the main argument for increasing the number of nuclear weapons involves Russia. This is, to say the least, a bit odd in the context of an administration that has promised to strike alliances with Moscow. But it also works through a conflation concerning Russian nuclear doctrine. Proponents consistently claim that Russia has adopted a ‘more aggressive’ posture when it comes to using nuclear weapons in the event of a conflict. Matt writes about Moscow’s ‘assertive’ nuclear strategy. What they’re talking about is Russia’s nuclear “de-escalation” doctrine:

De-escalation rests on a revised notion of the scale of nuclear use. During the Cold War, deterrence involved the threat of inflicting unacceptable damage on an enemy. Russia’s de-escalation strategy provides instead for infliction of “tailored damage,” defined as “damage [that is] subjectively unacceptable to the opponent [and] exceeds the benefits the aggressor expects to gain as a result of the use of military force.” The efficacy of threatening tailored damage assumes an asymmetry in a conflict’s stakes. Moscow reasoned when it adopted the policy that, for the United States, intervening on behalf of Chechen rebels (for example) might seem a desirable course of action for a variety of reasons. But it would not be worth the risk of a nuclear exchange. Russia, however, would perceive the stakes as much higher and would find the risk of a nuclear exchange more acceptable. Indeed, in the early 2000s, Russian military experts wrote that US interference in the war in Chechnya could have resulted in a threat to use nuclear weapons.

Matt describes the doctrine quite well in his policy paper:

In an effort to counter NATO’s aggregate conventional military superiority, Russia has placed an increased emphasis on nuclear weapons in its military strategy and doctrine over the past decade and a half. Russian strategy calls for limited nuclear “de-escalation” strikes on NATO targets in the event that it is on the losing end of a conventional war with NATO. The strikes would not primarily aim to destroy NATO military or civilian targets, but to signal Russian resolve and thereby shock NATO into suing for peace on terms favorable to Moscow.

Note what this is not. It is not a nuclear warfighting doctrine. Moscow is not signaling that it will, say, compensate for conventional weakness by targeting US conventional forces with tactical nuclear weapons. The idea here is not to be able to wage (and win) a limited nuclear war at lower rungs of the “ladder of escalation.” In warfighting doctrine, you care about having “escalation dominance”: the ability to ‘outgun’ your opponent at each level of nuclear escalation and therefore deter—or at least control—escalation.

Rather, Russian de-escalation doctrine is more like a nuclear tripwire. It signals that Moscow will:

Use nuclear weapons first;

Do so in the event of a threat to the existence of the Russian state—understood, in practice, as a regional conflict on Russia’s border that draws in the United States and in which Russia faces defeat; and

Engage in a limited number of nuclear strikes as a very dramatic way of showing that if the US pushes Russia any further then the result will be mutually assured destruction.

Why would more nuclear weapons, and more flexible nuclear options, be a useful response to nuclear de-escalation? Beats me. Matt’s own explanation only really makes sense if we’re talking about nuclear warfighting:

Deterrence, however, is in the eye of the beholder, and President Putin may not be deterred by the prospect of a conventional-only response, especially one that might take weeks or months to assemble and employ. Moreover, NATO could be quickly outgunned in such an approach if Russia continued to use nuclear weapons in repeated strikes. Furthermore, in the wake of a nuclear attack, the leaders of NATO countries, including the United States, would need to consider the precedent being set and broader Alliance commitments.

The whole point of nuclear de-escalation is that if the US continues to push Russia at all—whether conventionally or with nuclear attacks—Moscow will respond with a massive nuclear attack. In this context, responding by enhancing US nuclear warfighting capabilities isn’t simply a category mistake—it’s actively dangerous if it encourages Washington to press on after Moscow launches its demonstration strikes.

Third, advocates for increasing the US nuclear arsenal argue that it will deter countries like China and North Korea. It will show them that building more nuclear weapons is fruitless. The answer to this is rather simple. China currently has about 260 warheads. North Korea possesses between six and eight. Neither country threatens to close the gap anytime soon. China remains uninterested in doing so, which means that the biggest risk comes from a shift to a more aggressive American policy. Indeed, North Korea can never close the gap in any meaningful way, which suggests Pyongyang is not terribly sensitive to the relative nuclear balance with the United States.

Note that the country engaging in the most aggressive increase in its nuclear arsenal isn’t North Korea, China, Russia, or any of the other countries usually discussed in this context. It’s Pakistan.

In sum, there are good reasons to support some degree of US nuclear modernization. At the very least, we need to ensure that America retains the human capital and other infrastructure necessary to adapt to future changes in the nuclear-weapons environment. But there remains no good case for increasing the size of our nuclear arsenal.