The most common strategy is to increase the alliance’s margin of military advantage over North Korea. At best, this strategy will produce a fragile standoff marked by periodic crises, a constant risk of deterrence failure and little progress toward the goal of a nuclear-free peninsula. At worst, attempting to escape a mutual deterrence relationship with the regime by extending the alliance’s dominance undermines stability on the peninsula. The existing approach will incentivize North Korea to continually expand and improve its nuclear arsenal, adopt command and control arrangements that increase the risk of accidental escalation, and maintain doctrine and plans that increase the risk of intentional escalation.

To reduce the risk of nuclear use, the U.S.-South Korea alliance should adopt a strategy to maintain stability rather than degrade it. Though Washington, Seoul and Pyongyang have shown little interest in arms control, modest initiatives focused on North Korea’s tactical nuclear arsenal are the best way of moving beyond a fragile, unreliable standoff to a more stable peace.

A Rapidly Expanding North Korean Arsenal

Though North Korea will never be recognized as a “nuclear-weapon state” under the Nuclear Nonproliferation Treaty, it will remain a nuclear-armed state for the foreseeable future. Since Kim Jong Un took power after his father’s death in 2011, he has transformed the country’s nuclear arsenal from a rudimentary, symbolic retaliatory capability to one that is diverse, sophisticated and postured not only for retaliation against an allied attack on the regime but also to fight and win a limited conflict and to coerce the allies into terminating that conflict on his terms.

In recent years, the North Korean regime has pursued a puzzling strategy for the development of its nuclear forces. Resource-constrained middle powers tend to concentrate on perfecting and demonstrating the reliability of a small handful of missile systems, benefitting from economies of scale in procurement and training. By contrast, North Korea has demonstrated a large constellation of different options, including multiple missiles each at intercontinental, intermediate and short ranges. Currently, or in the near future, North Korea may be able to deliver nuclear weapons by air-breathing, ballistic, aero-ballistic and hypersonic-glide vehicles that are based on some combination of mobile transporter-erector launchers, submarines, silos, railcars and launchers attached to the seabed.

It is unclear why North Korea has adopted such an expensive acquisition strategy. In North Korea’s centralized regime, a diverse arsenal is unlikely to be the product of competing bureaucratic interests between services (as in the United States) or defense contractors (as in the former Soviet Union). It is more likely to be the product of a desire to (a) confound the alliance’s planning for reconnaissance and strike operations, (b) hedge against the chance that a small number of systems could be compromised by a technical fault or by allied cyber or kinetic attacks or (c) portray the regime as a sophisticated nuclear nation (either for prestige or to underscore U.S. and South Korean vulnerability to the arsenal).

Stability in an Asymmetric Deterrence Relationship

Though North Korea’s advancements are alarming, neither they nor allied responses will qualitatively change the fact that the United States and South Korea are vulnerable to a North Korean attack. While the expected damage that the regime could inflict on the United States and its allies is lower than other major nuclear powers, it is still significant and rising rapidly in both quantity and probability. Though U.S. policy will not formally recognize mutual vulnerability with North Korea, the U.S.-South Korea alliance is now in a deterrence relationship with North Korea.

The critical characteristic of a deterrence relationship is its stability. A deterrence relationship is stable to the extent that neither side has an incentive to initiate conflict with nonnuclear forces or to escalate a crisis to the nuclear level. The alliance’s deterrence relationship with North Korea is extremely asymmetrical, far more than the rough parity of the U.S.-Soviet balance or even the current U.S.-China relationship. Pyongyang’s marked inferiority — both in its ability to detect incoming attacks against its nuclear forces and its leadership and to control its forces during an attack — means that the regime has high incentives to use nuclear weapons early in a crisis. In an extremely asymmetric deterrence relationship, stability is inherently low.

Deterrence can contribute to stability if it can provide disincentives to escalation while also assuring both countries that they can preserve a margin of safety if they demonstrate restraint. Unfortunately, the U.S.-South Korea alliance increasingly interprets the requirements of deterrence in ways that degrade rather than enhance stability. The alliance has sought to escape or deny the existing deterrence relationship by maximizing asymmetry on the peninsula, including by signaling an intention to kill Kim and undertaking an infeasible effort to prevent any and all damage to the United States and South Korea through preemptive strikes and missile defense.

In a relatively symmetrical deterrence relationship, as with the Soviet Union during the Cold War, attempts to gain an advantage may have shifted the balance of expected damage, leading to ongoing competition in strategic forces, but they did not meaningfully affect Moscow’s incentives to use nuclear weapons first. In an extremely asymmetric deterrence relationship, these efforts provide Kim strong incentives to expand and diversify his nuclear forces, to demonstrate their capability in increasingly prominent ways until U.S. leaders acknowledge them, to plan to use them for a variety of retaliatory and warfighting missions early in a crisis, and to posture them in ways that allow for rapid first use even in the event his nuclear command and control system is disrupted. Because attempts to gain advantage increase Kim’s incentives to use nuclear weapons first, the alliance’s current defense policy does not promote stability but instability.

An allied defense policy focused on maintaining stability would instead recognize that North Korea can inflict unacceptable damage on the alliance and concentrate on reducing its incentives to carry out those attacks or to posture its forces in ways that could cause those attacks to occur unintentionally. It would begin with U.S. and South Korean officials publicly communicating a tacit acknowledgment of mutual vulnerability. To reduce North Korean incentives for destabilizing action, the alliance should maintain an equilibrium of forces on the peninsula and reduce its reliance on a “decapitation” strategy in order to allow Kim a rational expectation of his own security.

Arms Control Can Promote Stability

Arms control is the central instrument for countries seeking to maintain stability in their deterrence relationship. The United States and Russia valued past arms control agreements as ways to acquire valuable information about each other’s capabilities and to prevent the deployment of destabilizing systems. In an asymmetric deterrence relationship, arms control becomes both more important and more difficult. It becomes more important because the relationship is inherently unstable. It becomes more difficult because it is harder for the more powerful side to credibly commit to limit its capabilities — not only because of political or psychological reasons that currently keep the alliance from pursuing arms control but because its forces will always contain some capabilities needed to attempt counterforce and decapitation.

The Korean Peninsula could benefit from a series of arms control negotiations that are predicated not on North Korea’s rapid disarmament but on a limitation of its arsenal in ways consistent with stability. The allies could, for example, seek modest limits on the number, types, location or readiness of North Korea’s new tactical nuclear arsenal, which are the systems that stand the highest risk of initiating or escalating a nuclear crisis but are not necessary to deter the major attacks that could threaten the regime’s survival. In turn, the allies should recognize that these gains require not just monetary payoffs from lifting sanctions, but analogous limits on their capabilities or operations on the peninsula that could reduce Kim’s dependence on nuclear first use. A modest, reciprocal agreement that restricted North Korea’s tactical nuclear arsenal could not only reduce the risk and severity of conflict but may potentially initiate a series of negotiations that could reduce the chance of crisis onset and lead to more ambitious agreements later.

Neither Seoul nor Washington should expect any arms control negotiations to obviate mutual vulnerability. Limiting other types of North Korean nuclear systems — for example, intercontinental or sea-launched missiles — may have other benefits for reducing the risk of accidents, the expected damage North Korea could impose on certain targets or the fiscal requirements for the alliance. However, Pyongyang is unlikely to limit the systems that it sees as necessary to preserve mutual vulnerability and ensure the regime’s survival. Instead, limiting North Korea’s tactical nuclear weapons should be the alliance’s top priority as they are tasked not with increasing the aggregate quantity of expected damage — which is already unacceptable — but with providing qualitatively new missions for the arsenal that increase the risk of crisis onset or escalation. By decreasing crisis stability, North Korea’s tactical arsenal actually reduces the regime’s security.

Because the U.S.-South Korea alliance has not adopted stability as a guiding objective, its military posture has exceeded the requirements of deterrence and is maintaining an unnecessarily high risk of North Korean nuclear use. While the current strategy may well preserve a fragile and brittle peace, it carries a significant risk of failure. A durable and reliable peace on the Korean Peninsula, and the long-term goal of a nuclear-free North Korea, requires arms control.

Adam Mount is a nonresident senior fellow at the Federation of American Scientists.