Following a dangerous escalation of tensions last year, few could have envisaged the rapid turnaround in events witnessed so far amidst a flurry of high-level summit diplomacy. Although the complete denuclearization of North Korea remains a hypothetical scenario for now, its prospect would herald huge implications not only for inter-Korean relations, but also regional security dynamics in Northeast Asia. A key question, therefore, is how a process of denuclearization would impact on the U.S.-ROK alliance – an alliance which in October will commemorate 65 years since the mutual defense treaty was signed between the two sides in 1953.

North Korea's perceived military threat constitutes a key rationale for American military alliances with South Korea and Japan. The U.S.-ROK alliance is arguably at its strongest when North Korea is deemed to be the greatest threat. Accordingly last year, responding to a succession of missile and nuclear tests, President Moon Jae-in not only sanctioned the largest joint military exercises yet seen between the two allies, but also fast-tracked the installation of the contentious U.S. THAAD anti-missile system, at the expense of South Korea's relations with China.

Despite the ongoing diplomatic rapprochement between the two Koreas, a key stated principle of Moon's strategy is to maintain a "strong security posture on the solid ROK-U.S. alliance and national defense." But if and when a serious denuclearization process gets underway, it will inevitably bring into the question the nature of the U.S.-ROK alliance as well as entail adjustments to its role, mission, and capabilities. Indeed, North Korea has long objected to an alliance that it views as threatening to its security and the dismantlement of which is a long-held objective.

As part of implementing confidence - and security -building measures to reduce military and political tensions, further alterations can be expected regarding U.S.-ROK military drills. These may range from their downscaling, the reduction of U.S. troop participation, changes to the content of more "sensitive" exercises, as well as refraining from deploying strategic assets to the peninsula, including nuclear-capable submarines, aircraft carriers, and bombers.

Denuclearization and normalization of relations will also likely reinforce what is a longer term trend of a "rebalancing" of the alliance in terms of division of labor and responsibilities towards South Korea. Its defense spending has steadily increased over the last decade to nearly $40 billion in 2018, while its defense reform plans place increased emphasis on countering other potential threats, such as terrorism, cybersecurity, and territorial disputes. In terms of the alliance, it has bolstered its troop commitments, defense payments, and under the Moon government, seeks to accelerate the transfer of wartime operational control (OPCON) – a move that would signal that South Korea is no longer the junior partner in the alliance.

Notwithstanding the above, it remains to be seen what precisely North Korea will condition its complete denuclearization on, and whether this extends to a removal of U.S. forces from the peninsula. This would presently appear to be a red-line for both the U.S. and South Korea.

But should North Korea go so far as dismantling and disarming its nuclear assets, it is presumable that this would be accompanied by a peace treaty to replace the armistice agreement, which would make it more difficult to sustain the rationale for a presence of U.S. forces. North Korea knows this and which is why it has consistently pushed for a peace treaty. China too, as a signatory, will also be keen to address the U.S. military presence. Furthermore, it would also call into question other aspects of the security alliance including the U.S. provision of extended deterrence or so-called nuclear umbrella – the removal of which would also be presumably be a core demand by North Korea if a nuclear - free Korean Peninsula is to be realized as per the recent Panmunjeom Declaration.

But even if the configuration of the alliance might see changes, there are a number of reasons why the U.S-ROK alliance might be maintained in spite of denuclearization.

First, aside from its nuclear arsenal, North Korea maintains one of the largest standing armies in the world as well as possesses significant conventional and other capabilities, including short and medium-range missiles and bio-chemical weapons which threaten South Korean cities. It remains unclear for now if and how such capabilities will be addressed in denuclearization negotiations.

Second, U.S. long-term strategy envisages growing geostrategic competition with China and Russia. The 2018 national defense strategy accordingly outlines the need to strengthen alliances and maintain presence in what are termed critical regions. Beyond North Korea, therefore, the bigger picture is an increasingly assertive China and deepening U.S.-China rivalry, as currently witnessed over trade and the South China Sea.

Third, the U.S.-ROK alliance has evolved significantly beyond its original mandate. It is now a more comprehensive, "global" partnership with cooperation in numerous domains, including cybersecurity, climate change, and energy, among many others. Sharing common values, both have a vested interest in maintaining a liberal world order, even if the Trump administration's actions on a host of issues raises question marks.

Looking ahead, it would seem almost inconceivable that the U.S. would throw South Korea under the bus and unilaterally dismantle the alliance. It remains equally unlikely that South Korea, feeling safe, requests that it no longer desires a security alliance. It would thus take a fundamental shift in posture to contemplate its abandonment.

Such shock factors aside, if North Korea recedes as a threat as denuclearization proceeds, the cohesion of the alliance will likely be increasingly tested, as it was under the height of sunshine policy in the early 2000s. This could lead to increased debate within South Korea on the value of the alliance, especially if increasingly directed by the U.S. against China with whom South Korea will inevitably have to balance relations.

In the short term at least, the U.S.-ROK alliance will not change fundamentally. In the longer term, both sides will have to recalibrate their alliance commitments under the new geo-political situation of a denuclearized North Korea. But for now such a prospect still appears to be far off – let alone what unification would spell for the alliance.