It has long been a concern that North Korea’s nuclear posturing would goad or frighten its neighbours into developing their own nuclear capability. Now South Korea’s president, Yoon Suk-yeol, has raised the prospect of Seoul acquiring its own nuclear weapons. Yoon stated ^[1] last month: ‘It’s possible that the problem gets worse and our country will introduce tactical nuclear weapons or build them on our own. If that’s the case, we can have our own nuclear weapons pretty quickly, given our scientific and technological capabilities.’

At the same time, Yoon emphasised opportunities for nuclear sharing with the United States, stating ^[2]: ‘Currently, there is a discussion between South Korea and the United States in which we share information, participate together, jointly plan, and jointly execute the operation of these US nuclear assets’.

His office quickly clarified ^[3] that South Korea’s priority was to strengthen extended deterrence security guarantees to deter any threat from North Korea, a position that Yoon later confirmed. ‘I can assure you,’ he said ^[4], ‘that the Republic of Korea’s realistic and rational option is to fully respect the [nuclear non-proliferation] regime … I’m fully confident about the US’s extended deterrence.’

However, the door is now open to the possibility of South Korea developing nuclear weapons.

The threat from North Korea is front and centre in driving Seoul down this path. Pyongyang is seeking to rapidly develop tactical nuclear weapons ^[5] that can be used to threaten South Korea directly. Seoul can’t ignore that growing threat. A seventh ^[6] North Korean nuclear test remains likely and Pyongyang continues to develop ballistic missiles that can deliver tactical and strategic nuclear weapons. North Korean leader Kim Jong-un has called for ‘exponential’ expansion ^[7] of his country’s nuclear capabilities.

That threat is emerging against a background ^[8] of Russian nuclear threats against NATO, which many believe undermine traditional norms that de-emphasise nuclear weapons. The effects of Russia’s attempts at coercion on the resilience of US extended nuclear deterrence guarantees are no doubt being watched closely by Beijing, Pyongyang and perhaps even Tehran.

ASPI senior fellow Rod Lyon has highlighted ^[9] uncertainties about how US extended nuclear deterrence guarantees are applied, and there are concerns in Seoul and Tokyo about the resilience ^[10] of those guarantees in the face of threats from Moscow, and the prospect of such coercion by China in a future crisis over Taiwan. South Korea’s government is in effect asking whether the US will be prepared to risk trading Seattle or San Francisco for Seoul or Busan.

There’s no indication ^[11] that diplomacy between the two Koreas, or between the US and North Korea, will offer a path to comprehensive and verifiable denuclearisation of North Korea.

Added to this is the potential for a return of Donald Trump as US president after 2024, which must resuscitate concerns ^[12] in Seoul over whether Washington would continue to support the alliance even in the face of a growing nuclear threat from Pyongyang. A return to the ‘photo-op diplomacy’ that characterised discussions between Trump and Kim would again lead nowhere ^[13], or worse, result in the US making strategic concessions for little in return beyond empty ‘America first’ boasts to the media.

Yoon would also be paying attention to domestic political factors. Polling suggests that up to 70% ^[14] of South Koreans support developing an independent nuclear deterrent. Yoon can’t ignore such widespread support, especially as the threat grows from the North. Another North Korean nuclear test could quickly force his hand.

So, what happens if South Korea takes the plunge and acquires its own nuclear forces?

To get the bomb, Seoul would have to leave the Nuclear Non-Proliferation Tray, dramatically weakening it. North Korea left the NPT in 2003 to pursue nuclear weapons, but for a liberal democracy to follow suit would be a heavy blow to the treaty’s perceived role as the inviolable centerpiece of nuclear non-proliferation. Might Tehran then seize its own opportunity, and would other states follow suit? After all, if Russia were to demonstrate the effectiveness of nuclear threats and North Korea acquired many tactical nuclear weapons without cost, that would reinforce the legitimacy of nuclear weapons in the eyes of some actors. How many states walking away from the NPT would deal it a death blow?

For South Korea to acquire an independent nuclear deterrent would damage its relationship with the US, and with its neighbours, even if uncertainty over Washington’s extended nuclear deterrence commitments was a factor in Seoul’s decision-making. China would certainly demand some form of sanctions on Seoul, and politically it would be difficult for the US to avoid imposing any costs without its own credibility being undermined.

South Korea’s neighbours would have to respond. Japan has the technological skills to develop nuclear weapons relatively quickly, though constitutional constraints could slow the process. But Tokyo has been steadily moving away from the tight limits on its development of military capabilities, and the ruling parties’ recent decision to develop a counterstrike capability ^[15] based around land-attack cruise missiles has opened up a political window if Japan’s strategic environment deteriorates sharply. A breakdown in confidence in US extended deterrence would place immense pressure on Tokyo to act, especially if South Korea had taken that first step.

If South Korea and Japan were to both get nuclear weapons, it’s very likely that China would more accelerate the build-up of its own nuclear capabilities, perhaps moving faster towards its goal of 1,500 warheads by 2035 described in the Pentagon’s most recent China Military Power report ^[16]. Might China move away ^[17] from its ‘no first use’ policy? There’s also the question of tactical nuclear weapons for the Chinese military. The Pentagon report notes that China’s military commentators said in 2021 that precise small-yield nuclear weapons could be used for warning and deterrence, and suggested that they could lower the cost of war.

The emergence of South Korea, and potentially Japan, as nuclear-weapon states could encourage Beijing along that road.

Clearly a better option is to strengthen US extended nuclear deterrence by bringing Seoul into enhanced nuclear sharing arrangements, though those arrangements would need to be carefully defined. A nuclear sharing arrangement needn’t imply that South Korea would have operational control over US nuclear forces, which would make it significantly different to NATO’s nuclear sharing approach ^[18]. For example, it’s unlikely that US tactical nuclear weapons based in South Korea would be carried on South Korean military aircraft.

Likewise, Japan, and other key Indo-Pacific partners such as Australia, need to play a role—perhaps in an Asian nuclear planning group similar to that in NATO, as researcher Jennifer Ahan suggested ^[14] last year. An Indo-Pacific nuclear planning group would enhance collective decision-making over nuclear weapons and strengthen US extended nuclear deterrence guarantees.

Such steps could avoid an unconstrained proliferation cascade sweeping across Asia that would be highly destabilising and could lead to the collapse of nuclear non-proliferation norms.