Meanwhile, Japan appears to be laying the groundwork for a similar move. Speaking at a press briefing last Thursday, Japanese Defence Minister Shinjiro Koizumi said the country needed to re-evaluate its current submarine capabilities.

“The environment surrounding Japan is truly becoming so severe that we must debate whether to continue with diesel as we have, or to opt for nuclear-powered submarines,” Koizumi said.

Japan is already involved in Aukus projects related to maritime autonomous systems and AI-piloted aircraft. South Korea is in discussions to participate in research around hypersonic weapons and quantum military applications.

Economic, political factors at play

Nemeth noted that the US decision to share nuclear propulsion technology with South Korea could help smooth the path for Japan to pursue similar capabilities. But he cautioned that such support was far from guaranteed.

“It is not automatic,” he said, noting that Washington’s move appeared to be driven as much by industrial and economic considerations as by alliance politics.

“If Japan were to seek a similar arrangement, offering tangible contributions to bolster US shipbuilding capacity would likely make its case stronger,” Nemeth said.

He added that while Japan had the industrial and technological capacity to develop nuclear-powered submarines, it must obtain Washington’s consent to enrich and reprocess nuclear material under their bilateral nuclear cooperation agreement.

“In practice, Japan cannot move ahead independently without US consent,” Nemeth said.

Grant Newsham, senior research fellow at the Japan Forum for Strategic Studies, said that if Japan were to declare its intention to build a nuclear submarine and ask the US for help, Washington would likely agree.

“The idea of Japan having nuclear subs seems unobjectionable, if not sensible,” Newsham said. The US was so “overstretched” that Tokyo would likely have to pursue nuclear submarine development regardless of Washington’s thinking, he added.

“If Japan and South Korea build nuclear submarines, it will have a stabilising effect since they are more able to defend themselves against Chinese and North Korean aggression,” Newsham said.

“Aukus was more a joining of like-minded nations rather than ‘Anglo nations’ joining.”

Yoichiro Sato, a professor at Japan’s Ritsumeikan Asia-Pacific University, said that any nuclear-powered submarines of Japan and South Korea operating in Southeast Asian waters might run into “political opposition” in the region.

Jakarta’s concerns stemmed from the potential for a regional arms race, the weakening of nuclear non-proliferation norms, and the environmental impact of nuclear-powered vessels in its EEZ and archipelagic waters.

Unlike their conventional diesel-electric counterparts, nuclear-powered submarines have longer ranges and can operate further away from their home ports.

While Japan and South Korea have been well served by conventional submarines, they now face pressure to be “a useful ally” of Washington by integrating their plan into the US strategy to anchor the US commitment to their defence, according to Sato.

“President Trump has approved this historic step, and I want to reaffirm that decision,” Hegseth said in Seoul during a joint press conference with South Korean Defence Minister Ahn Gyu-back.

Adding that the US defence department would actively and fully support the decision, Hegseth said the nuclear submarine programme would strengthen the bilateral alliance as well as Seoul’s defence.

“South Korea has world-class shipbuilding capabilities, and the US looks forward to expanding cooperation not only in submarines but also in surface warfare,” he said.

Yang Uk, research fellow and director at The Asan Institute for Policy Studies’ Centre for Foreign Policy and National Security, said the possession of nuclear submarines would have significant implications for international politics and security in Northeast Asia.

“Nuclear submarines are an inherently geopolitical weapons system,” Yang said.

Such vessels combine stealth, endurance, global reach and strategic deterrence. They can remain submerged for months, making them highly resistant to a first strike and thus a stabilising element in nuclear strategy. By reducing the temptation for pre-emptive attacks, they help shape the strategic decision-making of major powers.

Deepening ties

“The risk that a deterioration in bilateral ties could lead to a naval clash – causing severe damage to both sides – could instead serve as a powerful incentive for maintaining stable and positive relations between South Korea and China,” Yang said.

Jennifer Parker, expert associate at the Australian National University’s National Security College, said many questions remained about how the project would work.

“We still don’t know what the submarine design will be or how the programme will be supported,” Parker said, adding that Seoul does not need nuclear-powered vessels in the same way Australia does.

“South Korea has a strong and modern navy, but nuclear-powered submarines are a major step up and will require time, infrastructure and deep specialist capability,” she said.

According to Parker, the initiative’s success will add to the proliferation of nuclear-powered boats in the region and also “strengthen the balance against China’s expanding nuclear submarine fleet”.

It would also “deepen defence ties between [Washington] and Seoul amidst some worries about America’s commitment to allies in general”, she said.

“So perhaps that inspired [Trump] to expand to others,” Le Thu said.

During their White House meeting last month, Trump and Albanese reaffirmed the Aukus agreement, with Trump declaring the deal would proceed “full steam ahead”, ending months of speculation about the future of the pact under his administration.

Yang added that South Korea had initially asked the US to provide nuclear fuel for use in its submarines, but Washington responded by suggesting that Seoul build its nuclear submarines at the Hanwha Shipyard in Philadelphia.

“This implies that the submarines will be constructed based on Korean-developed technology and Korean design” and not its American counterparts, he noted.

However, certain aspects such as reactor noise reduction could still make use of US technical support during the construction process, Yang added.

Washington shares its naval nuclear propulsion technology only within the Anglosphere: the UK, a long-standing partner, and Australia.

Yang said what made Aukus distinctive was that the two anglophone powers, the US and the UK, had chosen to support Australia’s ambition to play a greater role in the Indo-Pacific region.

However, under Trump’s second administration, scepticism was emerging over sustained US support for Australia, Yang said, noting that Japan was likely to seek access to US nuclear submarine technology similar to what Australia is receiving.

“Tokyo will be closely watching how South Korea proceeds with building its submarines within the US,” Yang added.