November 10, 2025
SINGAPORE – With US President Donald Trump giving South Korea the green light to build its own nuclear-powered submarine, a decision that has reignited debate in Japan on boosting its own nuclear defence capability, experts have raised concerns about escalating tensions and a potential arms race in the region.
They also foresee backlash from Russia, China and North Korea, all of which would not want the US allies to acquire more advanced underwater weapon systems that rival their military prowess in the region.
Mr Trump had announced on Oct 30 that the US will help South Korea build a nuclear-powered submarine.
This came a day after South Korean President Lee Jae Myung told him during a meeting in the south-eastern city of Gyeongju that Seoul’s current diesel-powered submarines have limited underwater navigation capabilities that restrict the military’s abilities in tracking North Korean or Chinese submarines within its waters.
The decision has since unleashed debate in Japan on whether Tokyo should follow in Seoul’s footsteps in asking for nuclear submarines, especially in the wake of a defence plan unveiled by newly minted Prime Minister Sanae Takaichi on Oct 21 that had included a push to acquire “next-generation submarines with advanced propulsion capabilities”.
Ms Takaichi’s policy speech dovetailed with a Defence Ministry expert panel’s recommendation on Sept 19 for Japan to acquire such submarines, marking the first time an official government advisory body has openly endorsed such an idea, signalling a potential seismic shift in Tokyo’s defence policy.
Both Japan’s and South Korea’s separate nuclear cooperation agreements with the US bind them to seek consent for any enrichment or reprocessing of uranium, which yields nuclear fuel.
The idea of nuclear energy being used in defence has long been anathema in Japan, given its unique status as the only nation to suffer direct nuclear attacks. Public opposition to nuclear weapons has remained strong for decades, deeply rooted in the lasting trauma still fresh in Japanese people’s minds.
Japan’s longstanding three non-nuclear principles, adopted in 1967, stipulate that the country will never produce, possess or host nuclear weapons.
But on Nov 6, Japanese Defence Minister Shinjiro Koizumi pointed to the worsening regional security environment in stressing that Tokyo must not rule out nuclear-powered submarines.
Nuclear-powered submarines are vessels that use a nuclear reactor for propulsion and may carry either conventional or nuclear weapons.
“All surrounding countries have them,” Mr Koizumi said on a news programme. “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.”
China’s nuclear-powered submarines are already said to operate in the East and South China seas, as well as in the western Pacific, while Russia’s submarines are present in the Sea of Okhotsk, north of Hokkaido.
Dr Andrew Yeo, a senior fellow and SK-Korea Foundation chair at the Brookings Institution’s Centre for Asia Policy Studies, warned that South Korea’s acquisition of a nuclear-powered submarine would not be taken kindly by North Korea, China and Russia, the three partners that have been dubbed the “axis of upheaval” for their opposition to the US-led global order.
Instead of bolstering security on the Korean peninsula, there is a “risk of instability and a possible spiralling arms race”, he told The Straits Times. Therefore, Tokyo’s biggest concern is likely the scenario of regional nuclear proliferation, he said.
Dr Yeo added: “Even though South Korea is not acquiring a nuclear weapon, its ability to process increased levels of uranium and acquire nuclear-powered submarines would create a more permissive environment for expanding nuclear capabilities, including nuclear weapons.”
South Korean Defence Minister Ahn Gyu-back has said that the country has the technology to build a nuclear-powered submarine in less than 10 years.
This would make it the eighth country in the world to be equipped with such submarines, after Russia, China, the US, France, Britain, India, and likely Australia.
Australia is set to acquire its first nuclear-powered submarines in the early 2030s, under the trilateral security framework between Australia, the UK and the US known as Aukus.
North Korea might also make its unofficial entry into the list soon, with its state media in March showing leader Kim Jong Un with a “nuclear-powered strategic guided missile submarine” that was under construction.
As part of Pyongyang’s doubling down on enhancing its nuclear strike capabilities, its five-year plan for national defence released in January 2021 had included a strategic goal of deploying nuclear submarines in the Korean peninsula.
Russia had likely handed North Korea two or three nuclear submarine modules salvaged from decommissioned Russian submarines in the first half of 2025, according to South Korean intelligence.
Under a mutual defence agreement inked in June 2024, Pyongyang has been supplying Moscow with artillery, missiles and soldiers to help Russia in the Ukraine war in exchange for fuel, food and military technology.
North Korea fired a ballistic missile off its east coast on Nov 7, days after US Defence Secretary Pete Hegseth’s visit to Seoul on Nov 3-4, during which Mr Hegseth made a pit stop at the Demilitarised Zone with South Korea’s Mr Ahn, where they “reaffirmed the strong combined defence posture and close cooperation” of the US-South Korea alliance.
In a statement published by North Korean state media on Nov 8, North Korean Defence Minister No Kwang Chol accused the US of “intentionally escalating the political and military tension in the region” and threatened “more aggressive actions against the threat of our adversaries”.
The regime had earlier fired cruise missiles on Oct 29, the day that Mr Trump arrived in Gyeongju. Despite the US leader’s repeated overtures to meet Mr Kim during his Asia trip, Pyongyang did not respond.
During his visit, Mr Hegseth told reporters that Seoul’s commitment to increasing defence spending would bolster its ability to take the lead in conventional deterrence against threats from the North, adding that Mr Trump’s decision to allow South Korea to have nuclear-powered submarines was driven by a desire to have strong allies.
Dr Bence Nemeth of the Defence Studies Department at King’s College London said the move strategically serves both Washington and Seoul.
“The US needs additional shipbuilding capacity – including faster production of nuclear submarines – while South Korea has long sought them. It looks like a win-win,” he told ST.
The nuclear-powered submarine approval was done on the sidelines of Seoul-Washington talks about South Korea’s pledged US$350 billion (S$455 billion) investment as part of its tariff deal, of which US$150 billion would be earmarked to revive the US’ flailing shipbuilding industry.
Mr Trump has announced that the submarine will be produced at the Hanwha Philly Shipyard in the US state of Pennsylvania.
The privately owned shipyard was acquired by South Korean shipbuilder Hanhwa Ocean for US$100 million in 2024, with a further $5 billion in investment pledged in August, and is seen as a shining example of Seoul’s commitment to rebuilding Washington’s shipbuilding industry.
But Seoul officials are negotiating with Washington for the vessel to be built domestically instead, given that the Hanwha Philly Shipyard currently lacks the necessary technology, workforce and infrastructure to build a nuclear submarine.
Hanhwa Ocean’s investments in the US have earned the wrath of China, which sanctioned five of its subsidiaries on Oct 14 for “assisting and supporting the US government’s probes and measures against Chinese maritime, logistics and shipbuilding sectors”, said a statement by China’s Ministry of Commerce.
This suggests that China-South Korea relations could face added strain as US-South Korea cooperation on submarines deepens, said Dr Nemeth.
And as Japan grapples with whether to join South Korea in the nuclear-powered submarine game, Professor Heng Yee Kuang of The University of Tokyo’s Graduate School of Public Policy thinks it may just help to better balance the powers in the region.
“It would enable the US and its allies to maintain their edge in the regional undersea balance of power,” said the analyst.