The Joint Comprehensive Plan of Action (JCPOA), reached by Iran, six other countries, and the European Union in Vienna in July, has sparked a heated political debate in the United States. Under the terms of the agreement, Iran has agreed to accept some temporary limits on its nuclear program in return for the lifting of the economic sanctions the international community imposed in response to that program. The Obama administration, a chief negotiator of the accord, argues that the deal will freeze and in some ways set back Iran’s march toward nuclear weapons while opening up the possibility of improving relations between the United States and the Islamic Republic, which have been bitterly hostile ever since the 1979 Iranian Revolution. The administration further contends that the agreement includes robust provisions for the international inspection of Iran’s nuclear facilities that will discourage and, if necessary, detect any Iranian cheating, triggering stiff penalties in response.

Critics of the deal, by contrast, argue that it permits Iran to remain very close to obtaining a bomb, that its provisions for verifying Iranian compliance are weak, and that the lifting of the sanctions will give Iranian leaders a massive windfall that they will use to support threatening behavior by Tehran, such as sponsoring global terrorism, propping up the Syrian dictator Bashar al-Assad, and backing Hezbollah in its conflict with Israel (a country that the Iranian regime has repeatedly promised to destroy).

To keep nuclear weapons out of Tehran’s hands will require a credible threat by the United States to respond to significant cheating by using force.

The American political conflict will come to a head in September, when Congress gets the chance to register its disapproval of the accord—although the president has promised to veto a disapproval resolution if it passes and has enough support among Democrats to uphold the veto and perhaps even to prevent a vote on such a resolution in the first place. Still, however the domestic politics play out, both the deal’s supporters and its critics agree that the United States should prevent Iran from getting a bomb. This raises the question of how to do so—whether without the deal, after the deal expires, or if the Iranians decide to cheat. Stopping Iranian nuclear proliferation in all three situations will require Washington to update and adapt its Cold War policy of deterrence, making Tehran understand clearly in advance that the United States is determined to prevent, by force if necessary, Iranian nuclearization.

A CREDIBLE THREAT

The English political philosopher Thomas Hobbes noted in Leviathan that “covenants, without the sword, are but words.” Any agreement requires a mechanism for enforcing it, and the Iranian agreement does include such a mechanism: in theory, if Iran violates the agreement’s terms, the economic sanctions that the accord removes will “snap back” into place. By itself, however, this provision is unlikely to prevent Iranian cheating. The procedures for reimposing the sanctions are complicated and unreliable; even if imposed, the renewed sanctions would not cancel contracts already signed; and even as the sanctions have been in place, Iran’s progress toward a bomb has continued. To keep nuclear weapons out of Tehran’s hands will thus require something stronger—namely, a credible threat by the United States to respond to significant cheating by using force to destroy Iran’s nuclear infrastructure.

U.S. Secretary of State John Kerry speaks with Hossein Fereydoun, the brother of Iranian President Hassan Rouhani, and Iranian Foreign Minister Javad Zarif, in Vienna, July 2015. U.S. State Department / Handout via Reuters

The term for an effort to prevent something by threatening forceful punishment in response is “deterrence.” It is hardly a novel policy for Washington: deterring a Soviet attack on the United States and its allies was central to the American conduct of the Cold War. Deterring Iran’s acquisition of nuclear weapons now and in the future will have some similarities to that earlier task, but one difference is obvious: Cold War deterrence was aimed at preventing the use of the adversary’s arsenal, including nuclear weapons, while in the case of Iran, deterrence would be designed to prevent the acquisition of those weapons. With the arguable exception of Saddam Hussein’s Iraq, the United States has not previously threatened war for this purpose and has in fact allowed a number of other countries to go nuclear, including the Soviet Union, China, Israel, India, Pakistan, and North Korea. Does the Iranian case differ from previous ones in ways that justify threatening force to keep Iran out of the nuclear club?

It does. An Iranian bomb would be more dangerous, and stopping it is more feasible. The Soviet Union and China were continent-sized countries that crossed the nuclear threshold before the U.S. military had the capacity for precision air strikes that could destroy nuclear infrastructure with minimal collateral damage. Israel and India, like the United Kingdom and France before them, were friendly democracies whose possession of nuclear armaments did not threaten American interests. Pakistan is occasionally friendly, is a putative democracy, and crossed the nuclear threshold in direct response to India’s having done so. The United States is hardly comfortable with the Pakistani nuclear arsenal, but the greatest danger it poses is the possibility that after a domestic upheaval, it could fall into the hands of religious extremists—precisely the kind of people who control Iran now.

North Korea presents the closest parallel. In the early 1990s, the Clinton administration was ready to go to war to stop Pyongyang’s nuclear weapons program, before signing an agreement that the administration said would guarantee that the communist regime would dismantle its nuclear program. North Korea continued its nuclear efforts, however, and eventually succeeded in testing a nuclear weapon during the presidency of George W. Bush. Since then, North Korea has continued to work on miniaturizing its bombs and improving its missiles, presumably with the ultimate aim of being able to threaten attacks on North America. It is worth noting that in 2006, two experienced national security officials wrote in The Washington Post that if Pyongyang were ever to achieve such a capability, Washington should launch a military strike to destroy it. One of the authors was William Perry, who served as secretary of defense in the Clinton administration; the other was Ashton Carter, who holds that position today.

Bad as the North Korean bomb is, an Iranian one would be even worse. For in the case of North Korea, a long-standing policy of deterrence was already in place before it acquired nuclear weapons, with the United States maintaining a strong peacetime military presence on the Korean Peninsula after the end of the Korean War in 1953. For this reason, in the years since Pyongyang got the bomb, its neighbors have not felt an urgent need to acquire nuclear armaments of their own—something that would be likely in the case of Iranian proliferation.

South Koreans watch file footage of a 1998 North Korean missile launch, Seoul, April 2009. Jo Yong-Hak / REUTERS

Nor would the Iranian case benefit from the conditions that helped stabilize the nuclear standoff between the United States and the Soviet Union. A Middle East with multiple nuclear-armed states, all having small and relatively insecure arsenals, would be dangerously unstable. In a crisis, each country would have a powerful incentive to launch a nuclear attack in order to avoid losing its nuclear arsenal to a first strike by one of its neighbors. Accordingly, the chances of a nuclear war in the region would skyrocket. Such a war would likely kill millions of people and could deal a devastating blow to the global economy by interrupting the flow of crucial supplies of oil from the region.

But if an Iranian bomb would be even worse than a North Korean bomb, preventing its emergence would be easier. A U.S. military strike against North Korea would probably trigger a devastating war on the Korean Peninsula, one in which the South would suffer greatly. (South Korea’s capital, Seoul, is located within reach of North Korean artillery.) This is one of the reasons the South Korean government has strongly opposed any such strike, and the United States has felt compelled, so far, to honor South Korea’s wishes. In the Middle East, by contrast, the countries that would most likely bear the brunt of Iranian retaliation for a U.S. counterproliferation strike—Saudi Arabia and Israel, in particular—have made it clear that, although they are hardly eager for war with Iran, they would not stand in the way of such a strike.

A LIMITED AIM

Deterring Iran’s acquisition of nuclear weapons by promising to prevent it with military action, if necessary, is justified, feasible, and indeed crucial to protect vital U.S. interests. To be effective, a policy of deterrence will require clarity and credibility, with the Iranian regime knowing just what acts will trigger retaliation and having good reason to believe that Washington will follow through on its threats.

During the Cold War, the United States was successful in deterring a Soviet attack on its European allies but not in preventing a broader range of communist initiatives. In 1954, for example, the Eisenhower administration announced a policy of massive retaliation designed to deter communist provocations, including costly conventional wars like the recent one in Korea, by promising an overpowering response. But the doctrine lacked the credibility needed to be effective, and a decade later, the United States found itself embroiled in another, similar war in Vietnam.

Iran’s ruling clerics may well doubt that Washington would actually follow through on a threat to punish Iranian cheating.

In the case of Iran, the aim of deterrence would be specific and limited: preventing Iran’s acquisition of nuclear weapons. Still, a policy of deterrence would have to cope with two difficulties. One is the likelihood of Iranian “salami tactics”—small violations of the JCPOA that gradually bring the Islamic Republic closer to a bomb without any single infraction seeming dangerous enough to trigger a severe response. The other is the potential difficulty of detecting such violations. The Soviet Union could hardly have concealed a cross-border attack on Western Europe, but Iran is all too likely to try to develop the technology needed for nuclear weapons clandestinely (the United States believes it has an extensive history of doing so), and the loopholes in the agreement’s inspection provisions suggest that keeping track of all of Iran’s bomb-related activities will be difficult.

As for credibility—that is, persuading the target that force really will be used in the event of a violation—this posed a major challenge to the United States during the Cold War. It was certainly credible that Washington would retaliate for a direct Soviet attack on North America, but the United States also sought to deter an attack on allies thousands of miles away, even though in that case, retaliation would have risked provoking a Soviet strike on the American homeland. Even some American allies, such as French President Charles de Gaulle, expressed skepticism that the United States would go to war to defend Europe. The American government therefore went to considerable lengths to ensure that North America and Western Europe were “coupled” in both Soviet and Western European eyes, repeatedly expressing its commitment to defend Europe and stationing both troops and nuclear weapons there to trigger U.S. involvement in any European conflict.

In some ways, credibly threatening to carry out a strike against Iran now would be easier. Iran may have duplicated, dispersed, and hidden the various parts of its nuclear program, and Russia may sell Tehran advanced air defense systems, but the U.S. military has or can develop the tactics and munitions necessary to cause enough damage to lengthen the time Iran would need to build a bomb by years, even without the use of any ground troops. The Iranians might retaliate against Saudi Arabia or Israel (whether directly or through their Lebanese proxy, Hezbollah), or attack American military forces, or sponsor acts of anti-American terrorism. But such responses could do only limited damage and would risk further punishment.

The administration should reinforce the credibility of its promise by increasing the deployment of U.S. naval and air forces in the Persian Gulf region.

The problems with deterring Iran’s acquisition of nuclear weapons are not practical but rather political and psychological. Having watched American leaders tolerate steady progress toward an Iranian bomb over the years, and then observed the Obama administration’s avid pursuit of a negotiated agreement on their nuclear program, Iran’s ruling clerics may well doubt that Washington would actually follow through on a threat to punish Iranian cheating. U.S. President Barack Obama initially embraced the long-standing American position that Iran should not be permitted to have the capacity to enrich uranium on a large scale, then abandoned it. He backed away from his promise that the Syrian regime would suffer serious consequences if it used chemical weapons. He made it the core argument in favor of the JCPOA that the alternative to it is war, implying that American military action against Iran is a dreadful prospect that must be avoided at all costs. Moreover, neither he nor his predecessor responded to Iran’s meddling in Iraq over the past decade, even though Tehran’s support for Shiite militias there helped kill hundreds of U.S. troops. The mullahs in Tehran may well consider the United States, particularly during this presidency, to be a serial bluffer.

DOUBT NOT

All of this suggests that in order to keep Iran from going nuclear, the JCPOA needs to be supplemented by an explicit, credible threat of military action. To be credible, such a threat must be publicly articulated and resolutely communicated. The Obama administration should declare such a policy itself, as should future administrations, and Congress should enshrine such a policy in formal resolutions passed with robust bipartisan support. The administration should reinforce the credibility of its promise by increasing the deployment of U.S. naval and air forces in the Persian Gulf region and stepping up the scope and frequency of military exercises there in conjunction with its allies. As in Europe during the Cold War, the goal of U.S. policy should be to eliminate all doubts, on all sides, that the United States will uphold its commitments.

The debate about the Iran nuclear deal has become politically polarized, but a policy of deterrence should not be controversial, since all participants in the debate have endorsed the goal of preventing an Iranian bomb. In addition, a robust policy of deterrence would help address some of the shortcomings of the JCPOA without sacrificing or undermining its useful elements. And since the deterrence policy could and should be open ended, it would help ease worries about the provisions of the accord that expire after ten or 15 years. As during the Cold War, the policy should end only when it becomes obsolete—that is, when Iran no longer poses a threat to the international community. Should the Islamic Republic evolve or fall, eliminating the need for vigilant concern about its capabilities and intentions, the United States could revisit the policy. Until then, deterrence is the policy to adopt.