A dramatic shift in the U.S.–South Korea defence partnership
Donald Trump announced that the United States has given South Korea approval to build a nuclear-powered submarine, signalling a major recalibration of the long-standing ally’s naval capabilities.
During his visit to South Korea, Trump posted on social media: “I have given them approval to build a Nuclear Powered Submarine, rather than the old fashioned, and far less nimble, diesel powered Submarines that they have now.”
The decision was made in conjunction with discussions held in Gyeongju ahead of the 2025 Asia‑Pacific Economic Cooperation Summit (APEC) and comes with broader trade and security agreements.
What the deal entails
- South Korea is authorised to build nuclear-powered submarines, potentially at the Philadelphia shipyard owned by South Korea’s Hanwha Group.
- South Korean President Lee Jae Myung made clear the submarines will be conventionally armed (non-nuclear weapons) but powered by nuclear reactors to boost underwater endurance and tracking capability.
- The announcement also opens the door for South Korea to seek U.S. assistance in acquiring nuclear fuel for submarine propulsion and possibly revising longstanding agreements that restrict South Korea’s nuclear-fuel reprocessing and enrichment.
Strategic motivations behind the move
- For the U.S., the expansion of South Korea’s submarine fleet with nuclear propulsion strengthens the alliance and helps share the underwater vigilance burden in a region facing growing maritime and submarine-based threats from North Korea and China.
- For South Korea, diesel-powered submarines require frequent surfacing to refresh power systems, limiting stealth and endurance. Nuclear-powered submarines remain submerged longer and are better suited for tracking adversaries’ undersea activities.
- The deal also ties into larger trade discussions: South Korea’s investment commitments in U.S. industries and energy purchases were part of the broader pact in which this defence move was embedded.
Key challenges and concerns
- Technology transfer and safeguards: Nuclear propulsion technology is among the most guarded by the U.S., last shared in the 1950s with the U.K. South Korea’s access marks a departure from precedent.
- Non-proliferation issues: Experts warn that allowing a new ally access to nuclear-propulsion technologies requires complex safeguards under the International Atomic Energy Agency (IAEA) and raises questions about enrichment and reprocessing capabilities.
- Regional reactions: China has already voiced concern, and the move could escalate undersea competition in East Asia, possibly prompting rival states to accelerate their submarine programmes.
- Implementation timeline: While approval is granted, many technical, legal and logistical hurdles remain — inclusive of building the reactors, securing fuel, and deploying subs — meaning operational capability remains years away.
What to watch next
- Whether the existing U.S.–South Korea nuclear energy cooperation agreement is formally amended to allow submarine fuel use and reprocessing/enrichment.
- The timetable for construction, where exactly the submarines will be built, and what reactors will power them.
- How North Korea and China respond — will this spark an undersea arms race or prompt new diplomatic initiatives?
- Whether this move influences other allies’ submarine ambitions, including those involved in the AUKUS framework (Australia, U.K., U.S.).
The bottom line
The green-lighting of South Korea’s nuclear-powered submarine programme marks a bold pivot in U.S. alliance dynamics and regional naval power structure. While symbolically powerful, the real world impact hinges on navigating complex nuclear-safeguard regimes and managing regional ripple-effects. The next few years will test whether this announcement becomes an operational reality or remains a grand diplomatic gesture.
