Sixty-nine years ago, an armistice ended three years of fighting involving the United States, China, South Korea and North Korea. In a speech marking the anniversary, North Korea’s supreme leader, Kim Jong-un, threatened ^[1] to use nuclear weapons and said his country was ‘fully ready for any military confrontation’ with the US and South Korea. That followed an April speech in which Kim implied that he’d consider pre-emptively using nuclear weapons, and his test ^[2] of an intercontinental ballistic missile—probably a Hwasong 15 ^[3]—on 24 March. Most significantly, North Korea looks set to undertake its seventh nuclear test ^[4] in coming weeks, which could coincide ^[5] with joint South Korea – US military exercises on the peninsula.

North Korea is also developing short-range, or tactical, nuclear weapons. Deploying them would lower the nuclear threshold on the peninsula and give Pyongyang the option of threatening first use to coerce its neighbour to make concessions. This is often referred to as ‘escalate to de-escalate’, particularly in discussions about how Russia might use tactical nuclear weapons in its war on Ukraine.

Nuclear policy expert Ankit Panda argues ^[6] that while North Korea may be acquiring tactical nuclear weapons for deterrence, they also could be used coercively to support brinkmanship and adventurism.

‘These North Korean developments have invited well-placed concern in South Korea and the United States that Pyongyang may explore the benefits of nuclear brinkmanship in future crises,’ Panda says. He notes that the alliance could be paralysed without credible retaliatory options should tactical nuclear weapons be fielded by North Korea.

In one scenario, instability in the north—perhaps brought on by a leadership struggle or economic collapse—could prompt Kim’s regime to generate a crisis near the demilitarised zone. Pyongyang could threaten to use tactical nuclear weapons to force Seoul to accept its demands, gambling that US extended nuclear deterrence would be neutralised by North Korea’s strategic nuclear forces. North Korea’s tactical nukes would become its sword and its strategic nukes the shield. Coercion and deterrence would be employed to maximum effect.

Would Washington risk a nuclear attack to defend a key ally under nuclear threat, or, as is often asked, would Washington be willing to trade Los Angeles for Seoul? If not, then extended nuclear deterrence falls apart, and US allies such as South Korea and Japan would likely seek their own nuclear deterrent capabilities against a North Korean nuclear threat or an aggressive China. Such a development could lead to a catastrophic broader collapse of nuclear non-proliferation as security dilemmas intensify. Non-nuclear states might respond by acquiring their own nuclear deterrents and nuclear states might decide to alter their nuclear force postures or expand their capabilities.

US and South Korean efforts to denuclearise North Korea seem unlikely to succeed, and there seems little that Seoul or Washington can do to dissuade Kim from further nuclear expansion, including deploying tactical nuclear weapons. The North Koreans have tested ^[7] delivery systems that can carry low-yield tactical nuclear warheads and may be set to resume ^[8] testing at the Punggye-ri site to inform the design of tactical nuclear warheads.

In May, South Korean President Yoon Suk-yeol and US President Joe Biden reaffirmed ^[9] their two countries’ commitments to defend each other under the 1953 mutual security treaty. Joint military exercises have been restored from a virtual command post to large-scale physical exercises on the peninsula.

The leaders’ statement should send a strong message to Pyongyang that Washington would fully support Seoul in a crisis. Biden affirmed the US extended deterrence commitment to South Korea using the full range of capabilities encompassing nuclear, conventional and missile defence. That suggests extended nuclear deterrence is still strong, even in the face of an emerging North Korean tactical nuclear threat.

In a practical sense, the US and South Korea have several options for countering tactical nuclear threats, including preventive and pre-emptive strikes against North Korean nuclear forces with precision conventional weapons. However, striking North Korean nuclear forces that are road-mobile is not likely to achieve overwhelming success in a crisis, and seeking to strike the North Korean leadership to prevent nuclear use may see Kim devolve launch authority to field commanders. Nor should reliance be placed on US ballistic missile defence. The US’s current national missile defence and sea-based ballistic missile capabilities provide, at best, a leaky umbrella, not an astrodome defence. That will be particularly the case against larger attacks as North Korea builds up its strategic nuclear forces.

Hence, it’s important that the Biden administration’s still-to-be-released nuclear posture review ^[10] consider how to respond to ‘escalate to de-escalate’ threats using tactical nuclear weapons, whether from Pyongyang or Moscow. That may require greater investment to accelerate development of conventional prompt-strike capabilities ^[11] based on hypersonic weapons or greater efforts to enhance the reliability of ballistic missile defence, including another look at boost-phase defence capabilities. Ultimately, there may need to be a debate on the size, posture and structure of future US tactical nuclear forces.

The Biden administration has sought to cancel ^[12] a proposed sea-based nuclear-armed cruise missile and would continue to rely on a small number of B61-12 ^[13] free-fall tactical nuclear bombs. But there’s already an imbalance ^[14] between US tactical nuclear forces based around the remaining B61-12s and the very large Russian tactical nuclear forces. North Korean tactical nuclear weapons add to this imbalance. China appears yet to go down the tactical nuclear weapons path but has invested ^[15] in regional nuclear capabilities. The yawning gap between US conventional forces and strategic nuclear forces could be exploited in an ‘escalate to de-escalate’ scenario, because it leaves the US with no choice but to respond to a use of a tactical nuclear weapon with a strategic nuclear response.

One option that’s been discussed is whether a build-up of North Korean tactical nuclear weapons might justify a deployment of US tactical B61-12 bombs onto the peninsula as a signal to Pyongyang that extended deterrence won’t falter in a crisis. Prior to his election, Yoon sought ^[16] that, suggesting a nuclear-sharing arrangement between US and South Korean forces.

An alternative would be for South Korea to develop ^[17] its own nuclear capabilities—an option that has strong popular support ^[18]. In a recent poll, 71% of South Koreans backed an independent nuclear deterrent, while only 56% supported the redeployment of US nuclear forces. So, if the US wouldn’t consider a redeployment, South Korea might acquire its own nuclear forces, even if US extended nuclear deterrence remains in place.