President Obama leaves the stage after speaking during a news conference at the Nuclear Security Summit in Washington in April. (Andrew Harrer/Bloomberg News)

Jeffrey G. Lewis is director of the East Asian Nonproliferation Program at the Middlebury Institute of International Studies at Monterey. Scott D. Sagan is the Caroline S.G. Munro professor of political science and senior fellow at the Center for International Security and Cooperation at Stanford University. This op-ed was adapted from an article that will appear in the fall issue of Daedalus.

President Obama, in his final months in office, is considering major nuclear policy changes to move toward his oft-stated goal of a world without nuclear weapons. One option reportedly under consideration is a “no first use” pledge, a declaration that the United States would not be the first state to use nuclear weapons in any conflict. While we think that such a pledge would ultimately strengthen U.S. security, we believe it should be adopted only after detailed military planning and after close consultation with key allies, tasks that will fall to the next administration.

There is, however, a simpler change that Obama could make now that could have as important, or even greater, benefits for U.S. security. The president could declare, as a matter of law and policy, that the United States will not use nuclear weapons against any target that could be reliably destroyed by conventional means.

This might seem like common sense, but current U.S. doctrine allows the use of nuclear weapons against any “object” deemed to be a legitimate military target. In 2013, the Obama administration did issue a guidance directing the U.S. military to “apply the principles of distinction and proportionality and seek to minimize collateral damage to civilian populations and civilian objects” and pledged that “the United States will not intentionally target civilian populations or civilian objects.”

This was a good step forward. But Obama’s guidance omitted an important legal concept derived from just-war doctrine — the “principle of necessity,” which suggests that war planners must use only the minimum amount of military force necessary to destroy a target. Ignoring the necessity principle leaves a loophole large enough to fly a nuclear-armed bomber through. To give one egregious example, although the U.S. military does not target civilian populations directly, following the principle of noncombatant immunity, it insists that it can legally target civilian airports in an adversary’s cities because they could be converted to military airports during a war — and there is no restriction in place against using nuclear weapons against such a civilian airport.

In addition, the U.S. military has expanded its definition of legitimate military targets from “war-supporting industries” (munitions factories, for example) to “war-sustaining industries” — a vague category that might include electric power plants or factories that provide tax revenue for the state.

This expansion has been useful, for example, in permitting U.S. conventional bombing of oil refineries controlled by the Islamic State. But it also means that the U.S. military planners can aim nuclear weapons at an increasing number of industrial targets in Russia, China and North Korea and still claim that any civilians killed were not “intentionally targeted” and were therefore only “collateral damage.” The resulting nuclear war plans — which could produce tens of millions of noncombatant deaths — are still deemed to be consistent with the principles of distinction and proportionality found in just-war doctrine and the law of armed conflict.

If the Obama administration insisted on a stricter interpretation of the principle of necessity, the U.S. military would be obliged to emphasize conventional deterrence and war-fighting and limit nuclear targeting to a greatly reduced list of hard targets, such as enemy missile silos or hardened command-and-control bunkers. There are some who argue that any limits on the United States’ freedom to use nuclear weapons automatically decrease the credibility of deterrence threats. But in most plausible scenarios, U.S. conventional military options are very effective; and because they are less apocalyptic and would not end the 71-year tradition of not using nuclear weapons, conventional strike options are more credible than nuclear options.

Most of all, we think the proposal conforms with good judgment and prudence. For it is hard to imagine a circumstance in which it would be either ethically responsible or strategically wise to use a nuclear weapon when a conventional one would suffice.

It is time to turn nuclear common sense into national policy. A declaration that the United States would never use nuclear weapons when conventional weapons could destroy the target could reduce the number of nuclear weapons we need for legitimate deterrence purposes. Placing conventional weapons at the center of debates about the future of deterrence would also help focus the policy discussion on plausible scenarios with realistic plans for the use of U.S. military power. And it would more faithfully honor the just-war principles of distinction, necessity and proportionality, by placing them at the heart of our deterrence and security policies, where our highest ideals belong.