In the 2010 review of U.S. nuclear posture, President Barack Obama’s administration, based on advice from military commanders and the extant global threat environment, concluded that the United States could ensure effective nuclear deterrence without fielding new nuclear warheads or warheads with new military capabilities. But even while foreclosing such options for the Obama administration, the 2010 review made clear, as did the nuclear posture reviews of the two previous administrations, that the nation must retain a capability to develop and field such warheads if they are required in the future.

Recently, a Defense Science Board report has caused a bit of a dustup in national security circles. Among other things, it calls for exploration of nuclear warheads with less explosive force — in the range of a few kilotons compared to the multi-hundred kiloton warheads prevalent in today’s arsenal. This recommendation is controversial in part because some see it as a repudiation of Obama’s position. It has emerged from a realization that the global threat environment has evolved significantly since the 2010 review.

Since then, Russia has rejected the post-Cold War order as reflected by its illegal annexation of Crimea and efforts to destabilize other sovereign states. President Vladimir Putin has not subjected Ukraine to an all-out armored assault as the Soviets did in Hungary in 1956. Rather, he has sought to achieve his political ends by introducing covert forces employing so-called “gray operations” to incite or amplify instabilities and insurgencies among fringe elements in Eastern Ukraine. That, and the progress that North Korea’s rogue regime has achieved in its nuclear weapons and ballistic missile programs, suggests that future major conflict involving the United States and its allies could play out very differently than previously thought.

This is not the Cold War scenario involving a massive global nuclear exchange. Rather, in a conventional conflict, an adversary could resort to limited nuclear use as part of a strategy to maximize gains or minimize losses. Some call this an “escalate to win” strategy. Limited use could be intended to consolidate territorial gains from an initial conventional attack by making it more difficult for the United States to come to the defense of allies. Or it could be intended to end a losing conflict short of regime demise. The Trump administration’s ongoing review of U.S. nuclear posture should explore options, including low-yield warheads, to counter this strategy.

Opponents argue that low-yield nuclear weapons blur the line between conventional and nuclear warfare, undermining deterrence by lowering the nuclear threshold and making nuclear war more likely. This assertion is not based in fact. In previous decades, the United States had thousands more tactical warheads than today, many with much lower yields. (By 1991, nearly all of these warheads had been retired from service and were subsequently dismantled.) The warheads were deployed at the height of the Cold War but never used even in intense regional conflicts such as Vietnam where U.S. use posed little risk of a nuclear response from Russia or China. There is no evidence that the mere possession of these weapons during the Cold War made the United States more likely to use them. Rather, these weapons were never used because nuclear deterrence worked.

Critics also argue that low-yield warheads are for warfighting, not deterrence, and once any nuclear weapon is used, escalation to a global holocaust cannot be controlled. Sen. Dianne Feinstein, responding to the Defense Science Board’s report on low-yield nuclear weapons, argued, “There’s one role — and only one role — for nuclear weapons, and that’s deterrence. We cannot, must not, will not ever countenance their actual use.”

Her statement, while well-meaning, reflects a fundamental misunderstanding. Deterrence is based on the enemy’s belief that the United States has both the capability and the will to employ nuclear weapons in extremis when vital national interests are threatened. A “threat to use” has, therefore, always been a part of the deterrence equation that has prevented any use of nuclear weapons for over 70 years.

Would an initial limited nuclear exchange escalate uncontrollably? Many Americans, including some in the nuclear policy community, believe that it would. If our nuclear-armed adversaries shared this belief, then it might not be necessary to consider low-yield nuclear weapons since the fear of all-out nuclear war would deter all parties from even limited use. Several, however, including Russia, as seen in recent doctrinal changes, modernization efforts and military exercises related to limited-use options, seem to believe that nuclear escalation could indeed be controlled.

Policymakers like Feinstein must remember that it is not what the United States believes that matters for deterrence — after all, we are not deterring ourselves. It is all about what the adversary believes. Exploration of low-yield options, therefore, is about deterrence, not warfighting. Only with a failure to deter, because a potential response is not credible, does U.S. nuclear use come into play.

To deter limited nuclear use, the United States should ensure a nuclear posture, declaratory policy, and set of flexible capabilities to convey to adversaries that no advantage, only unacceptable consequences, would result. What adversaries do or do not consider a credible response will always be uncertain. Consider, however, a hypothetical Russian low-yield strike on a European port that killed few but seriously disrupted U.S. plans to reinforce a Baltic ally under assault. Would the Russians believe that the United States would retaliate with multi-hundred kiloton warheads, creating the potential for substantial casualties? Would U.S. response be more credible if it had a broader spectrum of nuclear strike options?

As a result of such concerns, the United States has retained a few hundred low-yield B61 bombs for delivery on strategic bombers and NATO fighter aircraft. Ongoing modernization programs involving the B61, the new cruise missile, the B-21 bomber, and F-35 nuclear capability will preserve such options for the future.

U.S. strategic land- and sea-based ballistic missiles, however, do not have low-yield warheads. If they did, the United States could strike anywhere in the world, with greatly reduced unintended casualties, within tens of minutes of a president’s decision. This capability could be achieved with a small, relatively low-cost modification to existing warheads without requiring underground nuclear tests. It would help deter aggression by adversaries, for instance, by allowing the United States to place at risk, once located, mobile command posts highly valued by enemy leaders.

Finally, it’s critical that the United States assure allies of its commitment to come to their defense, including with nuclear weapons. Like deterrence, assurance is in the eye of the beholder and allies today are ever more mindful of the dynamic threats in their regions. Some, like South Korea, have shown interest in exploring an increased U.S. regional nuclear presence — potentially, because of collateral damage concerns, involving lower-yield warheads.

The nuclear reviews of the three previous administrations concluded that force numbers and capabilities mattered and that these could be adjusted as adversary behaviors, target sets, and employment doctrines evolved. As part of its ongoing review of U.S. nuclear posture, the Trump team, unburdened by myths and fallacies, should explore options to strengthen deterrence and assurance, including fielding a low-yield warhead for strategic ballistic missiles.

Prior to retiring from government service in 2013, Dr. John R. Harvey held senior positions in the Departments of Defense and Energy overseeing U.S. nuclear weapons programs serving, in his last assignment, as Principal Deputy Assistant Secretary of Defense for Nuclear, Chemical and Biological Defense Programs.

Image: Arnold Air Force Base/Rick Goodfriend