The rough road ahead for US-South Korea relations
The rough road ahead for US-South Korea relations
WRITTEN BY HUNTER SLINGBAUM
8 May 2025
These first several months of 2025 have been especially tumultuous for both the United States and South Korea. US President Donald Trump has spent the beginning of his second term upending longstanding policies and norms. One of his more recent, all-consuming focuses has been trade deficits and so-called “unfair tariff disparities”, which he addressed with sweeping “reciprocal” tariffs against nearly every other nation. One week later, and most of those tariffs were temporarily paused. His inconsistency and capriciousness have caused US allies to doubt the reliability of US security guarantees and extended deterrence, while also stoking concern for the world economy and free trade norms.
South Korea has also seen its fair share of internal turmoil, beginning with (now former) President Yoon Suk Yeol’s martial law declaration in December 2024. For the last four months, South Korean politics have been in disarray; the country is still without an elected leader. While the period of martial law itself was a flash in the pan, the fallout and existing fractures it revealed in South Korean society and politics are not easily salved.
Amid this uncertainty, US-ROK relations were sure to be tested. But Trump has given South Korean leaders additional reasons to be concerned. Experts fear that current interim leadership in Seoul may be pressured into sweeping concessions with long-term consequences.
In the coming months, South Korea should remain focused on its own presidential election and repairing its domestic fabric, but interim leadership can also explore ways to weather Trump’s demands, including by reframing existing efforts, as Canada did with their border plan.
Though South Korea has largely not been at the forefront of President Trump’s agenda, that is not to say that the US-ROK relationship has remained stagnant or purely speculative in the first stretch of the new administration. To best understand where we are now, it is crucial to examine three areas: the impacts of South Korean domestic turbulence, Trump’s global trade war, and the prospects of nuclear proliferation.
South Korea’s leadership crisis
South Korea’s current domestic instability and leadership turnover threatens to undermine its negotiating position with the US. In both his first term, and the start of his second, President Trump has fixated on the interpersonal side of US foreign relations, favouring strong leaders that have a firm handle on their constituents and are vocal in their praise of his leadership. Look no further than Japan’s former Prime Minister Shinzo Abe, who leveraged an active personal relationship with Trump in his first term to gain a number of symbolic and policy wins, including Trump’s adoption of Abe’s Free and Open Indo-Pacific (FOIP) strategy. Former South Korean President Yoon knew this well, privately and publicly congratulating then President-elect Trump and, soon after, returning to the golf course to practice his swing for future meetings.
With Yoon officially removed from office by the South Korean Constitutional Court, his successor will face the challenging task of building a relationship with Trump from scratch while attempting to stabilise the domestic political situation. The martial law declaration may have been the initiating event, but South Korea’s ‘leadership crisis’ has persisted in the ensuing four months, with the role of president changing hands three times. The first brought then Prime Minister Han Duck Soo into a short-lived stint as interim President. Barely two weeks later, he also faced impeachment and was replaced by his deputy, Choi Sang Mok. The latest turnover, Han’s reinstatement in March, will not endure either, as Yoon’s removal triggered preparations for the next presidential election, set to be held on 3 June.
This instability is not happenstance; it is a symptom of the polarisation that has infected South Korean politics and threatens the functioning of its democracy. The sheer number of both pro- and anti-Yoon protests since the martial law declaration serve as evidence of this deep divide. According to the ACLED, January 2025 saw a 61 per cent increase in pro-Yoon demonstrations compared to December 2024, and such protests have more closely matched rates of anti-Yoon protests since the start of February 2025. That is all to say, even when a new president is elected in the coming months, they will have a challenging future ahead in quelling domestic unrest, unifying a divided country, and presenting a strong image in future negotiations with the Trump administration.
The precipice of a (worse) global trade war
Trump’s assault on the global economy has been much more dramatic than most analysts anticipated beforehand, and it seems that few nations were spared. Before the pause, South Korea was slapped with 26 per cent reciprocal tariffs, one of the highest rates levied against a US treaty ally. As the Japan-US-ROK trilateral partnership falters, Chinese, South Korean, and Japanese trade ministers met in March for the first time in five years to promote regional trade and, according to some reports, are working toward an accelerated trilateral free-trade agreement. Given the challenging histories between these three countries, it would take a shared threat of significant magnitude to drive greater collaboration between them. To put it bluntly, things are getting serious.
Despite the current pause on most of the reciprocal tariffs, Trump’s attempts at aggressive economic coercion are likely to continue. Trump is fixated on trade deficits as a definitive signal of exploitation in US economic relationships, and, with the fifth largest US trade deficit in the Indo-Pacific, South Korea is unlikely to remain unscathed. In fact, the United States’ trade deficit with South Korea in 2024 was USD 66 billion, nearly USD 3 billion more than the US deficit with Canada, a country that has repeatedly faced Trump’s economic ire. An effective and persuasive head of state could possibly redirect Trump’s energy by reselling existing government efforts, such as those bolstering US ethanol exports to the country, and extensive direct investments by Korean companies in the US as ‘great deals’ or offering minor financial concessions. But South Korea’s current leaders seem poised to, at best, slightly delay the inevitable or, at worst, cave to increasingly onerous demands.
Nuclear ambitions back on the rise
That some in South Korea are considering acquiring nuclear weapons is hardly news; such calls have been bubbling back up to the mainstream since 2023. Public support for the development of an indigenous nuclear deterrent has slowly risen since 2014, with more recent polls suggesting that over 70 per cent of the population is in favour. Lack of support among the political elite has stymied any move toward nuclearisation, but there’s a convincing case that this time could be different.
Not only have more allies joined South Korea in considering nuclearisation in the wake of Trump era alliance shake ups, but the country’s internal calls have also gotten louder since President Trump was elected. Even a majority of the aforementioned political elite are willing to support nuclearisation in the case of US retrenchment, which Trump has considered in the past, and threatened if South Korea refuses to foot a USD 10 billion bill for US troop presence. And South Koreans are taking those threats to heart, noting Trump’s approach to Ukraine as a preview of what could be to come.
In other words, this longstanding alliance is showing signs of strain. South Korea is growing wary of the reliability of US security guarantees, its people increasingly support nuclearisation, and Trump is not afraid to strongarm allies, using American defence commitments as bargaining chips for economic wins. Adding salt to the wound, the US Department of Energy recently designated South Korea as a “sensitive country”, adding it to a list of nations that may pose a threat to US national security or to nuclear nonproliferation efforts. These factors hardly guarantee that US troops will leave South Korea, nor do they make South Korean nuclearisation a done deal, but a divergence of national interests that could eventually lead to a major fracture is clear.
Capitalising on change
Together, these diverging interests and complicating circumstances illustrate a US-ROK relationship that is in rough waters and bound to experience significant change in the coming months and years. Trump’s approach to foreign policy that favours economic gains above all else is likely to erode mutual trust and push Seoul to minimise its dependence on the United States. South Korea’s domestic turmoil and leadership crisis impedes its ability to firmly negotiate with the United States. That said, not all change is bad change, and there are some opportunities that policymakers in both countries should aim to capitalise on.
For one, some degree of US retrenchment in South Korea has the potential to be a net positive for both countries. For the United States, reduced commitments abroad mean less risk of entanglement in conflict, less threat to American lives, and more resources for domestic needs. For South Korea, US retrenchment would mean increased strategic autonomy, and, if executed correctly, still sufficient defensive capabilities. A recent report ranked South Korea’s military as the fifth strongest in the world, whereas North Korea ranked 34th. Concern about a deterrent gap can either be bridged with a continued promise of the US nuclear umbrella, or a slow, measured withdrawal while South Korea pursues its own capabilities.
More importantly, the Koreas have long been caught in a classic security dilemma, but tensions have been spiralling in recent years with increased instances of DPRK missile testing and joint US-ROK military exercises. The United States’ presence on the peninsula is, by its nature, threatening to the DPRK. South Korea, meanwhile, has pursued a more escalatory posture toward North Korea focused on preemption. If executed deliberately, US retrenchment in South Korea could cool these rising tensions by lowering North Korea’s threat perception and aligning South Korea’s diplomacy to its own capabilities.
A reduced US troop presence need not signal reduced US-ROK engagement; there are many areas in which the United States and South Korea can, and should, cooperate. For example, South Korea has long been at the global forefront of shipbuilding and could help rejuvenate shipyards in the United States. America’s largest military shipbuilder recently signed an agreement with a top South Korean manufacturer, a hopeful sign for the future. Beyond shipbuilding, South Korea’s burgeoning defence industry could fill other capacity gaps, including in technology and AI, cyber, space travel, and intelligence sharing.
In the coming months, South Korea should remain focused on its own presidential election and repairing its domestic fabric, but interim leadership can also explore ways to weather Trump’s demands, including by reframing existing efforts, as Canada did with their border plan. Government leaders should also support and develop the South Korean defence industrial base, both for exports and domestic capacity building. Lastly, while mutually beneficial collaboration with the US should be considered, South Korean policymakers should not be afraid to hedge with other partnerships, particularly amid doubts surrounding US reliability.
Likewise, the United States would benefit from continued engagement with South Korea. The Trump administration’s economic policies thus far have favoured protectionist measures, but leveraging allies’ comparative advantages is an efficient and dependable way to ensure that the United States becomes safer, stronger, and more prosperous.
DISCLAIMER: All views expressed are those of the writer and do not necessarily represent that of the 9DASHLINE.com platform.
Author biography
Hunter Slingbaum is a Research Assistant with the Reimagining US Grand Strategy Program at the Stimson Center. Their research primarily focuses on US foreign and defence policy in the Indo-Pacific, largely centring military strategy and air power. Outside of the military space, they also have research interests in the Korean Peninsula, diplomatic engagement, and cultural diplomacy. Image credit: US Army photo by Staff Sgt. Ken Scar.