How can the United States and South Korea best deter North Korea from carrying out a nuclear attack? In response to continued development of North Korea’s nuclear and missile capabilities, officials in Washington and Seoul have invested considerable effort to ensure that the ROK-U.S. alliance has a unified approach. Nevertheless, differences in their views persist, particularly over how best to communicate alliance deterrence posture. The most visible gap pertains to declaratory statements about how the United States would respond to a nuclear attack by North Korea.
The alliance has repeatedly stated that any such attack will “will be met with a swift, overwhelming and decisive response” and “result in the end of the Kim regime.” This statement does not say how the regime would end. U.S. officials have studiously avoided committing to the use of nuclear weapons in response to a North Korean nuclear attack. Instead, the alliance has reiterated that U.S. extended deterrence utilizes “the full range of U.S. defense capabilities, including nuclear, conventional, missile defense, and advanced non-nuclear capabilities.” This ambiguity in declaratory language reflects the fact that at the time a decision would need to be made about using weapons, leaders of the two countries would need to consider a range of specific circumstances that cannot be known in advance.
South Korean officials have urged the United States to take a more direct approach. While understanding the legal authority of the president of the United States, they argue that Washington should state clearly that North Korean nuclear first use would be met with a nuclear response. This approach tends to be referred to in South Korea as a “nuclear for nuclear” commitment. For example, prior to the April 2023 Washington summit between South Korean President Yoon Suk-Yeol and U.S. President Joe Biden, Korean media reported that “the South Korean government has reportedly conveyed its position that they hope the U.S. would respond with nuclear weapons if North Korea were to use them.”
To help government officials and experts understand what is behind these differences in views, we provide South Korean and American perspective on three questions about what actions and statements will best deter North Korea and why. These answers aim to characterize the mainstream beliefs of officials in Seoul and Washington rather than our own beliefs or those of any specific individual or institution.
Does ambiguity about how the alliance will respond to a nuclear attack strengthen or detract from deterrence of North Korean nuclear first use?
Sangkyu Lee and Suon Choi: The ambiguity of the United States’ response contained in its declaratory policy is clearly expressed in official documents such as the ROK-U.S. Security Consultative Meeting (SCM) joint press statements. At the 55th SCM in 2023, for example, ROK and U.S. officials expressed concern about North Korea’s advancement of its nuclear capabilities. Accordingly, the United States reaffirmed its commitment to provide extended deterrence to the ROK, utilizing the full range of U.S. defense capabilities, including nuclear, conventional, missile defense, and advanced nonnuclear capabilities. South Korean people interpret this U.S. statement as being ambiguous about the type of capabilities the United States would use in response to a North Korean nuclear attack, because it implies that even if North Korea uses nuclear weapons, the United States may still prefer to respond with only conventional weapons. For South Koreans, such an ambiguous declaratory policy raises concerns.
However, before discussing whether U.S. ambiguity will help deter North Korea from using nuclear weapons, it is useful to define some terms and assumptions. First, for the purposes of this discussion here, deterrence is defined as “the prevention of action by the existence of a credible threat of unacceptable counteraction and/or belief that the cost of action outweighs the perceived benefits.” Second, we assume that although North Korean leader Kim Jong Un may seem irrational, he will act rationally for his own benefit. Without defining deterrence and making this assumption, the uncertainty in predicting Kim’s behavior may become so great that the declaratory policies and deterrence measures of the ROK-U.S. alliance become meaningless. Another important assumption is that, as stated in the 2022 Nuclear Posture Review, the United States has the possibility of using nuclear weapons, including preemptive nuclear use, and that the ROK-U.S. alliance is capable of achieving escalation dominance over North Korea in all situations. Based on these assumptions, we focus on the extended deterrence cooperation developing between the United States and South Korea, rather than on the direct control of nuclear weapons, such as South Korea’s nuclear armament or the United States’ redeployment of tactical nuclear weapons.
Under an ambiguous declaratory policy, the possible conflict scenarios on the Korean Peninsula are (1) the U.S. responds to North Korea’s nuclear attack with nuclear weapons; (2) the U.S. responds to North Korea’s large-scale conventional, chemical, or biological weapon attack with nuclear weapons; (3) the U.S. responds to North Korea’s nuclear attack with conventional weapons; or (4) the U.S. responds to North Korea’s large-scale conventional, chemical, or biological weapon attack with conventional weapons. Kim might hesitate to use nuclear weapons if he believes there may be a highly destructive nuclear retaliation from the United States, as in scenarios one and two. In cases three and four, whether deterrence can be achieved depends on the perceived level of conventional response between South Korea and the United States. South Korean people think that if the lethality of a conventional response appears guaranteed, Kim would likely be afraid of this kind of retaliation as well, but if Kim misjudges that lethality is not guaranteed, he might not be deterred. These calculations point to why the assessed reliability of extended deterrence does not reach 100 percent in Korean public opinion polls.
Some South Korean experts and citizens believe that Kim’s nuclear use will be more strongly deterred if the United States commits to responding with nuclear weapons. Their concern is that a conventional response may provide space for Kim to misjudge U.S. resolve to employ nuclear weapons if necessary. And therefore, to reduce the possibility of misjudgment, these South Koreans prefer a clear U.S. declaratory policy that says a nuclear attack by North Korea will be met with a nuclear response.
Adam Mount and Toby Dalton: Most American experts and officials believe that an ambiguous declaratory policy is more credible than one that commits to nuclear use in a defined set of circumstances.
Though the United States avers that it would only use nuclear weapons in extreme circumstances to defend vital interests, it does not rule out nuclear use in certain circumstances against nuclear-armed adversaries. In the view of many Americans, retaining the option to use nuclear weapons first—to preempt a North Korean nuclear strike or to respond to extreme nonnuclear attacks—enhances deterrence because it forces Kim to weigh the risk of nuclear conflict in a range of cases. Successive presidents have declined to issue “sole purpose” or “no first use” statements because many American officials value this kind of ambiguity.
Ambiguity also enhances deterrence by preserving the alliance’s flexibility in responding to nuclear use, which allows the president to select the best military means to achieve the alliance’s objectives. Kim should understand that he will face an overwhelming response to nuclear use. The flexibility that comes from not specifying whether that response would be nuclear enhances deterrence because it forces Kim to consider a range of potential responses to nuclear coercion, which is more likely to induce restraint. American policymakers prefer policies that give them more options to deal with such circumstances, because they will want to select the option they assess is most likely to deter in any specific contingency.
American officials also hold a strong belief that nuclear threats must be credible. Issuing a general commitment to use nuclear weapons in response to a nuclear attack could weaken deterrence if it lacks credibility. In certain circumstances, an American president may conclude that a nonnuclear response is in the alliance’s best interest if, for example, a South Korean president argued against nuclear use; a North Korean nuclear detonation did not cause casualties because it exploded in the atmosphere, fizzled, or missed its target; a nuclear strike would be ineffective against certain targets or cause disproportionate collateral damage; or the alliance could inflict heavier costs with nonnuclear operations. Because they can imagine cases where a nonnuclear response is more effective at defending the alliance’s interests, they do not want to issue a blanket nuclear threat.
In general, American officials firmly believe that the United States does not and should not bluff about nuclear weapons. They value the credibility of American declaratory policy so highly that they will not threaten nuclear use unless they are absolutely sure they would carry it out. This kind of threat is only likely to come in the form of a presidential statement during a specific crisis. Because the president of the United States retains the sole authority to order nuclear use, any open-ended commitment such as “nuclear for nuclear” would lack credibility.
Americans worry that a general nuclear threat that lacks credibility in certain cases would degrade the credibility of all nuclear threats. Ambiguity therefore increases U.S. credibility for a time when officials need to issue an explicit nuclear threat in a crisis. They believe nuclear threats are more credible when they are rare, explicit, and specific and that an ambiguous declaratory policy in no way signals a lack of resolve to employ nuclear weapons if it is necessary to defend South Korea. In short, most American officials believe it is more important to maintain a reputation for doing precisely what they say than to try to cultivate a reputation that they would use nuclear weapons in a specific set of circumstances.
In one respect, the alliance has chosen to reduce ambiguity about how it will respond to nuclear use by declaring that any North Korean nuclear use will result in the end of the regime. Many American officials believe that clarity in this regard is valuable, though there is uncertainty about what this language entails in certain contingencies. The existing language still contains significant ambiguity about how and when the alliance would bring about the end of the regime.
In sum, American experts and officials believe that ambiguity about how to respond to nuclear use enhances the credibility of extended nuclear deterrence and is therefore the best policy to protect South Korea and the U.S. homeland from North Korean nuclear threats. Conversely, declaring a nuclear response without knowing the circumstances at the time is seen as irresponsible and counterproductive.
Is Kim Jong Un more deterred from first use of nuclear weapons by the expectation of a nuclear response than a nonnuclear response?
Sangkyu Lee and Suon Choi: Deterrence works through a combination of physical and psychological factors. If the alliance convinces Kim that a nuclear strike would be followed by a U.S. nuclear response, he will inevitably hesitate to use nuclear weapons. Threat is often interpreted as a combination of capability and willingness. The world, including North Korea, would not doubt the United States’ nuclear capability. However, there are likely to be different interpretations of whether the United States has the will to use nuclear weapons. Therefore, a clear statement of U.S. willingness to use nuclear weapons would increase North Korea’s fear and strengthen deterrence.
South Koreans believe that a U.S. nuclear response would be more effective than a conventional response in deterring North Korea’s preemptive use of nuclear weapons because they think that if Kim believes that the alliance will respond with nuclear weapons, he is less likely to launch a preemptive nuclear strike in the first place because he believes his losses would be too great. North Korea has reacted more sensitively to the flexing of U.S. nuclear strategic assets—such as during the visit of the nuclear-capable ballistic missile submarine (SSBN) in the ROK’s Busan Port in July 2023 and during periodic exercises of B-2 and B-52 strategic bombers on the Korean Peninsula that same year—than to the deployment of other conventional assets. This North Korean reaction makes South Koreans assess that Kim is more fearful of U.S. nuclear strategic assets. Conversely, if North Korea perceives that the United States and South Korea will respond to its nuclear use with a conventional response, it may decide that the benefits of nuclear provocation are greater and therefore decide to actively use nuclear weapons.
The Washington Declaration and joint press release from the April and December 2023 Nuclear Consultative Group meetings emphasize that the United States and South Korea are firmly committed to extended deterrence with the full range of U.S. capabilities, including nuclear, and that North Korea will face a “swift, overwhelming and decisive response” that will result in the end of the regime. Although the joint statements do not specify a U.S. nuclear response in all circumstances, they do emphasize the extended deterrence commitment, including the possibility of a U.S. nuclear response, which is a result of South Korea’s concerns about the efficacy of deterring a North Korean nuclear attack. More importantly, in order to deter North Korea’s preemptive nuclear use, the alliance must emphasize on multiple occasions that the alliance is making decisions without gaps and that cooperation continues to be solidified.
Adam Mount and Toby Dalton: Among American experts, there is no consensus about how Kim perceives the alliance’s capabilities. There is little evidence to indicate that he has a specific fear of nuclear weapons. Most American experts assume that Kim is likely most deterred by a range of scenarios that specifically threaten regime change.
One school of thought holds that the unequalled destructive potential of nuclear weapons means that they are likely to have a unique ability to deter an adversary. In this theory, nuclear weapons can inflict more damage more promptly than other forms of military power. This certainty should dissuade an adversary from thinking they can escape or resist a nuclear response. Some Americans also believe that nuclear threats can evoke a sense of terror that enhances deterrence.
Another school of thought holds that, in most or all circumstances, the alliance’s conventional capabilities are sufficient to produce an overwhelming response and would be preferable to nuclear use. According to this view, the threat of an overwhelming conventional response may even be a more credible deterrent in some circumstances because it would allow the alliance to strike more targets without harming civilian populations, mitigate the risk of drawing China or Russia into a conflict, and reinforce the taboo on nuclear weapons use. Kim should recognize the destructive potential of allied nonnuclear options. From the perspective of Pyongyang, it should not matter whether it is conventional or nuclear weapons that end the regime, decapitate its leaders, or destroy its forces.
Many American strategists will start from these kinds of intuitions, and nearly all are likely to believe that the credibility of a deterrence posture or the efficacy of a response will depend on the characteristics of a specific contingency. In deterring or responding to nuclear use, there is no “one size fits all” option. For example, conventional options may be more credible in responding to low lethal coercive demonstrations, while nuclear threats may be more effective in deterring mass casualty attacks.
Consistent with a desire to maximize flexibility, U.S. military posture provides the alliance with nuclear and nonnuclear options across a full range of contingencies.
Does the United States’ desire for ambiguity in its declaratory policy imply a lack of resolve, either to use nuclear weapons if necessary or to defend South Korea?
Sangkyu Lee and Suon Choi: In Question 2, we stated that because deterrence is fear-based, North Korea would be less likely to use nuclear weapons if it perceived that this use would be met by a U.S. nuclear response. Here, we discuss how U.S. ambiguity affects South Korea’s fear of North Korean nuclear use. For several reasons, South Korea may believe that the United States’ desire for ambiguity in its declaratory policy means Washington lacks the will to use nuclear weapons and the will to defend South Korea.
First, in the event of a North Korean nuclear attack, South Korea, which does not possess nuclear weapons, would seek the United States’ guaranteed assurance and consultation on a nuclear response. Since ambiguity does not guarantee a nuclear response from the United States, South Korea may perceive this as a lack of U.S. willingness to defend South Korea. It is well understood that both South Korea and the United States currently have an advantage over North Korea in terms of conventional forces. Therefore, in the event of a nuclear attack by North Korea, South Korea wants the United States to provide assurance around a strong nuclear response rather than around a conventional response. Notably, the South Korean public has a stronger desire for “a nuclear response for a nuclear attack” than policymakers and elites do. Thus, the United States’ refusal to assure South Korea of a nuclear response could be seen in South Korea as a weakening of U.S. willingness to defend South Korea. In contrast, the U.S. declaratory policy of responding with nuclear weapons without ambiguity would send a clear message that the United States is willing to use nuclear weapons and is strongly committed to defending South Korea.
Second, the fact that the United States is considering nonnuclear means is likely sending a message that its willingness to defend South Korea is weak. A decisionmaker like the United States may prefer a wide range of options to a limited set of options, but for allies that rely on U.S. assurances, having multiple options can be a source of anxiety. This is because the United States may choose a stronger or weaker response than its allies would prefer. In a scenario where South Korea would expect a nuclear response from the United States but the United States would still consider nonnuclear means, South Korea could perceive this consideration as an indication of a weaker U.S. willingness to respond.
Third, ambiguity increases uncertainty about the means with which the United States will respond, which may increase South Korean anxiety about an escalating crisis. In managing escalation, the need for close coordination between alliance partners becomes even more important. However, ambiguity raises uncertainty by increasing the number of variables that must be considered in a joint response, which makes South Korea more anxious. Furthermore, North Korea’s use of nuclear weapons is more likely to occur if it separates or decouples the alliance, capitalizing on South Korea’s anxiety. Thus, if the United States maintains nuclear response ambiguity, the possibility of conflict escalation and South Korean anxiety increase.
Adam Mount and Toby Dalton: No American official or expert believes that ambiguity in U.S. declaratory policy implies a lack of resolve to defend South Korea. For Americans, ambiguity is the policy that is most effective at deterring a nuclear attack.
In fact, American officials would be puzzled by the idea that there is a close link between nuclear declaratory policy and the United States’ commitment to defend South Korea. The terms of the ROK-U.S. mutual defense treaty are unconditional. For U.S. officials, the resolve to defend South Korea is not diminished by the vulnerability of the U.S. homeland to North Korean nuclear missiles, Chinese coercion, or any other factor. An alliance constitutes an enduring promise to defend an ally, even in the face of nuclear blackmail—especially in the face of nuclear blackmail. U.S. officials are willing to escalate a crisis if it is necessary to defend the interests of the United States and its allies. North Korean nuclear threats or nuclear attacks would likely increase, not decrease, the resolve of U.S. officials to defend the alliance.
For the most part, American strategists craft declaratory policy to send signals to adversaries rather than to allied publics. American officials largely expect that allied governments will help to shape public opinion in their countries in support of policies agreed within the alliance, including by affirming the credibility of the alliance’s nuclear and nonnuclear capabilities. American officials believe that their investment in nuclear assurance measures—including visits with strategic assets and high-level consultations—is evidence of their commitment to South Korea and their resolve to use nuclear weapons, if necessary, to defend it.
The preferences of allied governments have had an important influence in developing U.S. declaratory policy, including by steering policy reviews away from “no first use” and “sole purpose” proposals. The importance that U.S. officials place on these consultations with allied governments can hardly be overstated. However, U.S. officials will resist requests for policy changes that they believe will reduce the flexibility needed to safeguard the alliance’s best interests, including proposals that threaten an automatic or symmetrical nuclear response.
Many South Koreans are understandably concerned about political volatility surrounding the 2024 U.S. presidential election. However, a more stringent nuclear declaratory policy is not an effective hedge against unpredictability in subsequent U.S. administrations. A future president would not feel bound by past agreements on declaratory policy—and if that president did, it would not be in either country’s interests to prioritize a nuclear response to North Korean actions over defense with conventional forces. Thankfully, the overwhelming balance of national security officials and U.S. Congress members will remain steadfastly committed to U.S. allies, which means that, in practice, no single president is capable of abrogating the ROK-U.S. alliance.