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One side wants more explicitness in the level of commitment to the relationship, while the other wants to remain ambiguous.
While this tug-of-war could describe what some romantic couples experience, it can also describe the tension in the U.S.-South Korean alliance over the issue of Seoul’s confidence in U.S. extended nuclear deterrence. Today, a crucial difference in perspective persists between Washington and Seoul regarding the ideal degree of explicitness in U.S. nuclear security commitments and the specific means of achieving them. This divergence, stemming from differing policy preferences and rooted in a disparity of trust between a non-nuclear ally and a nuclear-armed security guarantor, will likely remain despite some improvements over the previous Yoon Suk Yeol administration. However, furthering mutual understanding of each other’s position and establishing supplementary institutional mechanisms would likely strengthen trust, tamper calls for South Korean nuclearization, and mitigate the destabilizing consequences of nuclear disagreements between the two longtime allies.
The Sources of Divergence
Over the past few years, the United States and South Korea have held divergent views on how specific U.S. extended deterrence commitments should be. Washington’s position can be summarized as “more explicitness may be worse.” The core reason behind this view is the concern that overly explicit commitments could constrain its freedom of action and increase the risk of entrapment in a nuclear war. An automatic nuclear retaliation pledge in response to a North Korean nuclear attack on South Korea could preclude the United States from exercising other options, such as large-scale conventional retaliation or decapitation strikes against the Kim regime leadership.
On the other side, Seoul believes “more explicitness will be better.” South Korea views clear and specific U.S. nuclear commitments as essential to precluding North Korean misjudgment and demonstrating Washington’s unwavering willingness to protect South Korea. In fact, Seoul’s insistence on explicitness is more pronounced than that of other U.S. allies. This is primarily because South Korea faces a direct and explicit nuclear threat from a neighboring country. As part of its five-year nuclear modernization plan, which began in 2021, North Korea has accelerated the development of nuclear forces aimed at South Korea, including non-strategic nuclear warheads, nuclear-capable cruise missiles, and tactical nuclear attack submarines. These developments have amplified concerns within South Korea that North Korea would employ nuclear weapons preemptively, offensively, and actively as a warfighting tool.
Diverging Preferences for Declaratory Policy and Action Policy
To understand the emergence of the structural divergence between two treaty allies, it is worth considering, as Paul H. Nitze contends, how U.S. nuclear extended deterrence policy has two connected components. The first is the declaratory policy, which refers to official statements or doctrines that articulate intentions, principles, or conditions regarding the use or non-use of nuclear weapons for defending allies. The second is the action policy, also referred to as operational policy, which translates declaratory policy into concrete operational measures. This policy governs the actual planning, deployment, and potential employment of nuclear forces to safeguard allied security.
Regarding the declaratory policy, Seoul worries that Pyongyang may perceive U.S. ambiguity about when it would use nuclear retaliation as a lack of resolve. From Seoul’s point of view, the best way to prevent North Korean miscalculation is a clear declaration: “Any nuclear attack will be met with nuclear retaliation.” Strategic elites in South Korea frequently insist the United States must pledge retaliation regardless of the nature of the North Korean strike — whether it targets only South Korea or includes the U.S. homeland, whether it is a warning shot or full-scale attack, and whether it precedes or follows conventional conflict. They argue only an explicit and specific nuclear pledge can credibly deter nuclear and conventional threats backed by a nuclear shield.
In contrast, the United States consistently avoids specifying the exact conditions under which it would carry out nuclear retaliation. From Washington’s perspective, this calculated ambiguity bolsters deterrence by forcing Pyongyang to weigh the risk of nuclear conflict across various scenarios — even in cases where the United States is not actually considering nuclear use. Accordingly, while reaffirming nuclear guarantees, the Biden administration maintained the traditional U.S. ambiguity. For example, the 2022 Nuclear Posture Review states that any North Korean use of nuclear weapons against U.S. allies is “unacceptable” and will result in “the end of that regime,” but it stops short of explicitly committing to nuclear retaliation.
Regarding the action policy, the Yoon administration feared that undisclosed extended deterrence plans would not only lead North Korea to doubt American willingness to uphold its nuclear commitments, but would also cause operational cleavages within the alliance itself. This latter point was particularly important given Seoul’s sustained investment in conventional counterforce capabilities. Such divergences, according to South Korea, would not only weaken the alliance’s combined defense and deterrence posture but could also incentivize North Korea to test this posture during a crisis.
However, the United States resists making detailed commitments, fearing that such specificity would enable North Korea to anticipate and prepare effectively for its responses. Instead, U.S. ambiguity increases Pyongyang’s uncertainty about how retaliation would unfold — where, when, and by what means — forcing it to grapple with multiple scenarios. This “operational unpredictability” can make Pyongyang more cautious about attacking South Korea or, if it decides to invade, compel the Kim regime to disperse forces in rear areas to guard against unpredictable U.S. counterattacks, thereby weakening its offensive punch.
An Unbridgeable Gap?
Uncertainty over the U.S. extended deterrence commitment emerged as a central topic in U.S.-South Korean relations under the Yoon administration from May 2022 to April 2025. The Yoon government considered the level of U.S. explicitness inadequate and sought more concrete and tangible measures by the United States to shore up South Korean confidence. Park Jin, Yoon’s first foreign minister, clearly articulated this concern in a February 2023 media interview, stating, “We need to discuss with the U.S. the execution of extended deterrence in the event of an emergency in more detail.”
In response, the Biden administration acknowledged Seoul’s concerns and initiated changes to U.S. declaratory and action policies through close cooperation with the Yoon government.
Regarding declaratory policy, in 2023 President Joe Biden made an unprecedented public statement in a joint press conference with Yoon, that a North Korean nuclear attack on South Korea would result in the end of the North Korean regime. In terms of action policy, the two governments established the Nuclear Consultative Group as part of the Washington Declaration endorsed in April 2023. They also advanced conventional-nuclear integration efforts and adopted joint guidelines on nuclear deterrence and nuclear operations. Finally, the nuclear ballistic missile submarine USS Kentucky made a port visit to South Korea in July 2023. All these moves aimed at reassuring Seoul and the South Korean public by clarifying the substance of U.S. nuclear commitments.
Nevertheless, many South Korean politicians, experts, and the general public remained dissatisfied with such measures, calling for more decisive steps to address what they considered continued U.S. ambiguity. Two steps were proposed. The first was the removal of any uncertainty in U.S. declaratory policy by the adoption of a “nuclear attack protection clause” in the alliance treaty, stipulating automatic U.S. nuclear retaliation if North Korea carried out a nuclear strike on South Korea. The second — on the action policy side — was the introduction of a NATO-style nuclear sharing arrangement, potentially involving the permanent (re)deployment of U.S. nuclear assets to or near the Korean Peninsula, such as in Guam.
Both current and former U.S. officials have called these proposals excessive. Regarding Seoul’s call for a “nuclear-for-nuclear” retaliation clause, a former senior State Department official referenced Biden’s public warning about regime-ending consequences for Pyongyang as an unprecedented move, asking pointedly, “If this isn’t an example of strategic clarity, then what is?” Washington’s overriding concern is that such a bold declaratory policy could reduce its flexibility for making future moves and increase the likelihood of entrapment in a conflict with a nuclear-armed state.
In addition, Washington views the submarine visit and the regular deployments of nuclear-capable bombers as clear demonstrations of U.S. extended deterrence. U.S. officials are disinclined to permanently station nuclear forces on or near the Korean peninsula. They argue that such deployments could unnecessarily escalate tensions and undermine the “operational flexibility” of the U.S. nuclear posture. As Vipin Narang, then acting assistant secretary of defense for space policy and co-chair of the Nuclear Consultative Group, noted in an interview, the United States doesn’t “assign specific weapons to specific missions or targets. If you pre-assign anything, then you’re reducing flexibility.”
An Enduring Challenge for the Trump-Lee Era
The ambiguity-explicitness gap will likely remain a source of tension between the two allies in the Trump-Lee era. During his successful June 2025 campaign for the South Korean presidency, Lee Jae Myung acknowledged the rapid expansion and growing sophistication of North Korea’s nuclear capabilities. In his inaugural address, Lee reaffirmed that his government would “maintain strong deterrence against nuclear threats and provocations” based on the alliance. This suggests an effort to make the U.S. commitment and its implementation measures more concrete while simultaneously taking a pragmatic approach toward inter-Korean relations.
Strong public support for an independent nuclear capability and persistent skepticism of the U.S. nuclear umbrella are likely to provide a powerful incentive for the Lee administration to push for more explicit commitments from Washington — not only to bolster deterrence against North Korea but also to address domestic political pressures. These pressures will only increase should North Korea reveal further developments in its nuclear program. Moreover, while South Korean strategic elites broadly oppose indigenous nuclearization, many, especially conservatives, still advocate the necessity of introducing a NATO-style nuclear sharing arrangement and redeploying non-strategic nuclear weapons to South Korea, arguing that these measures would resolve U.S. ambiguity and restore the credibility of its extended deterrence. The uncertainty surrounding the U.S. commitment is likely to remain a key theme in South Korean strategic discourse under the Lee administration, as it was under his predecessor.
The Way Forward
It is essential for both countries to manage the persistent challenge of explicitness versus ambiguity prudently to reduce unnecessary tensions. South Korea should acknowledge the limit to which it can coerce the United States into being less vague. A failure to do so will likely result in deeper resentment of the United States and louder calls in South Korea for developing an indigenous nuclear capability.
South Korean policymakers should avoid becoming fixated on the assumption that greater explicitness will necessarily lead to greater effectiveness of U.S. extended deterrence. The Lee administration should adopt a transparent, public-facing approach by issuing official statements and conducting public briefings to explain the inherent constraints on the level of clarity Washington can provide. Such an approach could help reduce public distrust in U.S. extended deterrence and alleviate political pressure for indigenous nuclearization. Seoul could then focus on strengthening existing joint initiatives established during the Biden-Yoon era.
For the United States, it is crucial to communicate candidly with the Lee administration that it cannot offer the level of specificity that Seoul seeks — and to clearly explain the reasons for this limitation. The Trump administration should emphasize that this does not signal a lack of resolve, but rather that a measure of ambiguity can be strategically advantageous in deterring a North Korean nuclear attack by inducing caution in its behavior.
Two institutional measures may help to narrow the explicitness-ambiguity gap. The first is creating a joint deterrence simulation center in South Korea, co-managed by U.S. and South Korean military officials. This center would utilize advanced simulation technologies to model real-time scenarios of North Korea’s potential nuclear and conventional attacks. By leveraging both countries’ advanced AI capabilities, the center could accurately estimate scenario-specific damage levels, North Korean attack methods and movement routes, as well as optimal allied response strategies and force configurations. Such realistic and detailed simulations would enhance South Korea’s confidence in the credibility of the U.S. commitment.
The second measure is establishing within the existing Nuclear Consultative Group a formal, service-specific consultation mechanism between corresponding branches of the U.S. and South Korean militaries (e.g., army-army, navy-navy, and air force-air force). Such detailed, service-level interaction would help South Korean forces gain a clearer understanding of U.S. extended deterrence policies, including the degree of strategic commitment and likely courses of action in a crisis. It would also support more informed bilateral coordination on the conventional-nuclear integration initiative by aligning operational concepts and expectations across corresponding branches.
South Korea faces an intractable, nuclear-armed adversary in North Korea and while the United States may feel that it has worked hard to reassure its ally, these efforts should evolve in line with further changes in Pyongyang’s nuclear posture. Although the United States is unlikely to alter the inherent ambiguity in its declaratory and action policies, a failure to continuously reinforce the alliance relationship in this strategic context will only lead to increased internal pressure for South Korea to go nuclear.
Do Young Lee is a postdoctoral fellow (assistant professor) in the Department of Political Science at the University of Oslo and a fellow at the Oslo Nuclear Project.
Ian Bowers is a professor at the Norwegian Institute for Defence Studies and an adjunct researcher for the Oslo Nuclear Project.
Image: U.S. Department of State via Wikimedia Commons
Bridging the Ambiguity-Explicitness Gap in the U.S.-South Korean Alliance – War on the Rocks
