In their December 4 National Interest article, Adam Lowther and Hunter Hustus assert that, when it comes to nuclear weapons, “less is not just less, less is different”, in order to cast doubt on the wisdom of reducing U.S. strategic nuclear forces below the level set by the New Strategic Arms Reduction Treaty (New START). “Less” may become “different” at some point, but it is hardly the case now, when the United States and Russia each have at least ten times as many nuclear weapons as the country with the third largest nuclear arsenal.

Lowther and Hustus note that the U.S. nuclear arsenal exists not just to deter attack on the United States but also to extend deterrence to U.S. allies in Europe and Asia. Extended deterrence is a more demanding task: an adversary may very well believe the United States would respond with nuclear weapons to an attack on American territory, but that same adversary may question whether Washington would so readily use nuclear weapons to defend an ally if that raised a risk of a nuclear attack on the United States.

Lowther and Hustus write that the Iranian and North Korean nuclear programs could create pressures on other countries, e.g., Saudi Arabia, to acquire their own nuclear weapons. These pressures would increase if doubts over the credibility of the American extended deterrent grew. Extended deterrence is not just about deterring the adversary but, equally importantly, about assuring allies.

Lowther and Hustus make serious points. But they jump from there to conclude that further U.S. nuclear reductions would be dangerous. Their logic in doing so is less clear.

According to the Federation of American Scientists, the U.S. and Russian nuclear arsenals currently each number about 4,500 total nuclear weapons (operational strategic, operational non-strategic, and reserve/non-deployed). The nearest third-country forces, France or China, possess about 300 or 250 nuclear weapons, respectively. This would appear to leave significant room for further U.S. and Russian reductions.

In a 2012 book, Michael O’Hanlon and I suggested a new agreement to limit the United States and Russia each to no more than 2,000-2,500 total nuclear weapons. We also envisaged a sublimit of 1,000 deployed strategic warheads and a separate ceiling of 500 deployed strategic missiles and bombers (down from New START’s limits of 1,550 deployed warheads and 700 missiles and bombers).

The resulting nuclear force would certainly be “less” but not necessarily all that “different.” The U.S. nuclear arsenal would number around eight times as many weapons as the arsenal of any third power. The United States could maintain a nuclear triad (one potential arrangement could be 160 deployed submarine-launched ballistic missiles on ten or twelve submarines, three hundred intercontinental ballistic missiles, and forty nuclear-capable bombers). This would leave plenty of room for non-strategic nuclear weapons.

To be sure, deterrence and extended deterrence are complex concepts that involve considerations beyond just numbers of weapons. They require getting into the mind of a potential adversary and understanding his calculations, motivations and fears, as well as what he values—so that U.S. nuclear forces can hold those things at risk and dissuade the adversary from certain actions. What may deter one adversary may not prove so successful with another. And deterrence needs may evolve over time.

Lowther, Hustus and I undoubtedly agree on these theoretical points. But let’s move to the real world. Which potential adversary would act differently if the United States had “only” 2,500 nuclear weapons instead of the 4,500 weapons at present? Would North Korea adopt a more aggressive posture toward the United States or toward nearby U.S. allies South Korea and Japan if the United States had just three hundred times as many nuclear weapons as it, instead of five hundred times as many? That is very hard to believe.

Deterrence conservatives often accuse those in the pro-arms control community of focusing too narrowly on numbers of nuclear weapons. But those who argue that any further reductions in U.S. nuclear force levels will undermine deterrence are often guilty of the same charge.

Extended deterrence (and assurance) in particular depends on far more than numbers. It concerns the confidence that allies have, especially in a crisis, that the United States will be prepared to employ its full military arsenal, including nuclear weapons, in their defense. Historical links, broader relationships, and personal trust between leaders have a major impact on that confidence.

If an ally’s leadership is absolutely, positively convinced that an American president will use nuclear weapons in their defense, no matter what, a U.S. nuclear arsenal of one thousand or fewer might well suffice for the purposes of assurance.

Too often, in addressing the challenges of extended deterrence and assurance, the United States has resorted to hardware solutions—such as the 1979 decision to deploy Pershing II and ground-launched cruise missiles in Europe—for what is largely a software problem—can we answer the question in an ally’s mind of Washington’s readiness to act? The United States continues to retain some two hundred nuclear bombs in Europe today even though few officials in Washington see a deterrence requirement there (in contrast to an assurance need).

Other approaches can contribute to addressing the challenge. Consulting with allies helps. Creation of the NATO Nuclear Planning Group in 1966 has provided a venue to give allies insights into U.S. (and British) nuclear planning, doctrine, and targeting as well as the opportunity to express their views. That software fix bolstered allied confidence in the U.S. nuclear deterrent.

Actions also have impact. As tension peaked on the Korean peninsula last spring, the U.S. Air Force flew B-52 and B-2 heavy bombers over South Korea. The overflights sent Pyongyang a pointed public reminder of U.S. capabilities and of the fact that South Korea lies under the U.S. nuclear umbrella.

Likewise, three days after China’s unilateral declaration of an air defense identification zone (ADIZ) over the East China Sea, two U.S. B-52s transited the zone. They disregarded the announced Chinese requirements for foreign military aircraft to notify flight plans in advance and maintain radio contact while in the ADIZ.

These actions sent important signals to South Korea and Japan of U.S. resolve.

There are strong arguments for further reductions in nuclear weapons below current levels and the limits of New START. The United States can pursue those cuts and still deter threats against itself and its allies, while also reassuring those allies. To be sure, Washington has to manage the reductions process in a smart way. But while “less” may become “different” in the future, we are far from that point now.

Steven Pifer is director of the Arms Control and Non-Proliferation Initiative at The Brookings Institution.

Image: Flickr/James Vaughan. CC BY-SA 2.0.