If it is to endure, the US–South Korea alliance must evolve from a narrow, cost-sharing security pact into a multidimensional strategic partnership.

The global order is at an inflection point. For South Korea and the United States, the alliance must be reshaped to safeguard future national security interests. Three imperatives demand urgent attention: a shared threat perception, a joint deterrence strategy, and a new strategic understanding of the alliance.

A Shared Korean-American Threat Perception

Washington understandably regards China as the most comprehensive and grave challenge. Beijing boasts a growing nuclear arsenal, projected to reach 1,000 warheads by 2030; the ability to project power into the Second Island Chain of the Indo-Pacific; a range of coercive threats toward Taiwan; and the means to carry out increasing “gray-zone” tactics against its neighbors. Seoul regards China with some suspicion as well; however, its main national security priority is North Korea’s nuclear and missile arsenal, expected to surpass 200 warheads by 2030. Traditionally, South Korea has confined its threat perception to the peninsula, with some political leaders even downplaying Beijing’s importance to South Korean national security.

This divergence is dangerous. North Korea and China cannot be separated as threats. In a Taiwan contingency, Pyongyang is likely to simultaneously stir up trouble on the Korean Peninsula, diverting Washington’s resources and creating the risk of a two-front war. Beijing’s and Pyongyang’s nuclear and conventional threats are interlinked. The United States and South Korea must, through sustained high-level consultations, forge a common understanding of this integrated threat environment.

A Joint Strategy Must Follow Shared Threat Perceptions

Historically, the United States provided nuclear extended deterrence to South Korea, while Seoul primarily focused on strengthening its conventional forces on the peninsula. Since 2023, the Nuclear Consultative Group has integrated US nuclear assets with South Korea’s conventional capabilities. Deterrence has also expanded to include Japan, recognizing that Pyongyang’s missiles pose a threat to the wider region.

The older notion of “strategic flexibility,” allowing US Forces Korea to deploy elsewhere, is outdated. That concept assumed a China still focused on economic growth rather than military assertiveness. Today, Beijing pursues territorial revisionism, gray-zone coercion, and “wolf warrior” diplomacy. Accordingly, deterrence must be both regional and multi-domain—integrating nuclear, conventional, space, cyber, and intelligence. US forces in Korea should not be diluted or drawn down, but rather enhanced with ISR, space, and information warfare capabilities to deter Chinese coercion directly on the peninsula.

The Trump administration is known for its skepticism of foreign entanglement, and President Donald Trump reportedly mulled pulling troops from the Korean Peninsula during his first term, though he did not ultimately do so. Before Trump resolves to try again, he must recognize two hard truths. First, North Korea fears the United States far more than it fears South Korea. Any weakening of US deterrence in Korea risks emboldening Pyongyang to launch integrated nuclear-conventional operations. Second, if the United States’ commitment to South Korea erodes, it is virtually inevitable that Seoul will seek its own nuclear weapons. To prevent this, the US may need to redeploy tactical nuclear arms to Korea, with shared cost burdens.

The US-South Korea Alliance Must Go Beyond Security

It must move beyond narrow cost-sharing debates toward broad benefit-sharing. The Trump-era disputes over defense burden-sharing reduced the alliance to zero-sum arithmetic. Yet the strategic gains from joint deterrence against China and North Korea far outweigh cost disputes. The new metric should be the collective benefits of deterrence, not transactional dollars.

For its part, South Korea must embrace self-strengthening defense and deterrence. Like its NATO allies, Seoul should considerably raise defense spending—to at least 3.5 percent of GDP in the near term. Enhanced resources should fund five priorities: acquisition of advanced US ISR satellites, joint development of nuclear-powered submarines, bolstering missile defense with US technologies, co-development of AI-enabled weapons, and partial funding for redeployment of US tactical nuclear weapons if extended deterrence requires it.

Expanding the scope of the alliance is equally vital. Beyond simply a security pact, Washington and Seoul should harness their complementary strengths in shipbuilding and nuclear energy. In particular, a US-South Korea shipbuilding alliance would strengthen both nations. America lags far behind China in naval and commercial shipping capacity. Korea’s advanced shipbuilding sector can fill this gap, with agreed Korean investment exceeding $150 billion in shipbuilding. This will enhance allied naval power and industrial resilience. The two countries should also cooperate on nuclear energy; together, they can pursue joint uranium enrichment to decrease reliance on Russia, co-development of Small Modular Reactors (SMRs), and coordinated entry into global nuclear markets to counter Chinese and Russian dominance.

In short, if it is to endure, the US–South Korea alliance must evolve from a narrow, cost-sharing security pact into a multidimensional strategic partnership. Shared threat perception, integrated deterrence, and a benefits-based calculator will ensure resilience. Expanding into shipbuilding and nuclear cooperation will elevate the alliance from a five-dimensional to a seven-dimensional framework—encompassing security, economy, technology, information, culture, shipbuilding, and nuclear energy. A renewed vision will allow the alliance not only to manage current challenges but to shape the future Indo-Pacific order.