South Korea has been a trusted ally of the United States for over seven decades, growing in importance each year as East Asia becomes a region of increasing importance for the United States. However, the lasting nature of this partnership risks complacency: If either state becomes too comfortable with the alliance, they could fail to understand changing dynamics or public sentiments of the other and therefore fall short on the commitments that hold the partnership together.

History of the U.S.–South Korea Alliance

The 1953 Mutual Defense Treaty codified the defense alliance between the United States and South Korea that began with the North Korean invasion of South Korea in 1950.REF Article III of the treaty states:

Each Party recognizes that an armed attack in the Pacific area on either of the Parties in territories now under their respective administrative control, or hereafter recognized by one of the Parties as lawfully brought under the administrative control of the other, would be dangerous to its own peace and safety and declares that it would act to meet the common danger in accordance with its constitutional processes.REF

Since the Korean War, this clause has helped deter aggression from North Korea. In addition to the 26,206 American military personnel in Korea,REF the United States also maintains a nuclear umbrella over South Korea,REF meaning that if South Korea comes under strategic attack—to include a nuclear attack or overwhelming conventional force—then the United States reserves the right to defend South Korea with all instruments of its national power, including with nuclear weapons.REF

While North Korea remains a real and growing missile and nuclear threat in both the Pacific theater and increasingly the American homeland, a new opponent has taken the center stage: China. Indeed, the United States is in a cold war with China, one that could turn hot in the near future.REF

If the U.S.–South Korean alliance is going to endure and remain meaningful in the coming decades—just as it has for the better part of a century—then both Washington and Seoul need to understand one another.

South Korea’s Perspective

First, Washington needs to understand that unless dramatic action is taken soon, Seoul may become a nuclear power in the coming years. If the United States fails to assure its allies that Washington’s nuclear umbrella remains credible, then those allies will seek alternative nuclear guarantees.REF

During the Cold War, the United States forward deployed non-strategic nuclear weapons to South Korea to deter communist aggression within the theater.REF Following the end of the Cold War, the United States withdrew the last of its nuclear weapons on the Korean Peninsula as part of President George H. W. Bush’s Presidential Nuclear Initiatives.REF Despite North Korea detonating its first nuclear weapon in 2006,REF the United States withdrew its nuclear Tomahawk non-strategic nuclear weapons from active service under President Obama.REF

Today, North Korea has upwards of 80 nuclear weaponsREF and a ballistic missile force that is increasingly capableREF and could target not only sites in Korea and Japan with nuclear weapons but also sites in North America. At the same time, China is the fastest growing nuclear power on the planetREF and is expected to reach nuclear parity with the United States by 2035.REF Beyond simply numbers of nuclear warheads, China is fielding a force that is increasingly capable of engaging in theater, low-yield nuclear warfighting across the Indo–Pacific—to include weapon systems that are ideally targeted to engage and destroy bases in Korea.REF

Indeed, the South Korean population increasingly supports the redeployment of American nuclear weapons to South Korea. Over time, this support has grown from 23.2 percent in 2016REF to 57 percent in 2022.REF Even beyond that, many South Koreans now support an indigenous South Korean nuclear weapons capability. The number of South Koreans desiring nuclear weapons has polled as high as 70 percent,REF and it was a key campaign issue in the latest presidential election.REF

During the Cold War, Seoul seriously pursued nuclear weapons when it believed that the United States might leave the theater in the aftermath of the Vietnam War.REF It halted its nuclear program only after the United States threatened to publicly terminate its defense commitment to South Korea.REF These concerns about American abandonment—particularly in the face of growing nuclear threats in North Korea and China—are clearly again putting security concerns at the forefront of South Koreans’ minds. Increasingly, South Koreans look at the deteriorating security environment and once again question if the United States’ nuclear umbrella is indeed credible.

Every year, the American and South Korean military forces participate in the Freedom Shield training exercise.REF After the 2025 exercise, North Korea responded with frustration, referring to Freedom Shield as a “war rehearsal.”REF North Korea claimed that the exercise was justification for further expansion of North Korean nuclear capabilities—and continued with its missile testing efforts.REF

Next Steps for the U.S.–South Korea Alliance

South Korea’s desire for self-protection is not illogical. Sharing a border with a hostile adversary that is expanding and maturing its nuclear forces—and engages in near-continuous nuclear threats against it—would be cause for concern for any country. Washington should take these genuine fears seriously. If Washington wants to keep South Korea from becoming a nuclear power, it should assure Seoul that its security guarantee—to include its nuclear umbrella—is serious and meaningful.

While the United States has done much to assure South Korea in recent years—from engaging in the aforementioned military exercises to bomber assurance and deterrence missionsREF to port visits by American ballistic missile submarinesREF to the U.S.–South Korea Extended Deterrence Strategy and Consultation GroupREF—these efforts are demonstrably no longer sufficient to assure Washington’s allies in Seoul.

What Washington needs to understand, then, is that if it wants to prevent South Korea from once again seeking nuclear weapons, Washington should seriously consider once again forward stationing nuclear weapons on the Korean peninsula—just as it did in the Cold War—as a means to both deter communist aggression and prevent its regional allies from getting their own nuclear weapons.

At the same time, Seoul should understand that if it wants to remain a relevant and valuable partner to Washington, it should expand its aperture to become a major power within the Pacific theater.

South Korean policymakers should understand that the United States faces risks in sharing nuclear assurances. This arrangement, important as it is, exposes the American homeland to potential attack. Indeed, North Korean dictator Kim Jong Un has said repeatedly that the United States coming to the aid of South Korea makes American cities legitimate targets for nuclear attack.REF While targeting population centers is a war crimeREF and would undoubtedly result in an overwhelming nuclear response from the United States, the fact remains that the United States puts its homeland at risk of nuclear attack due to its security commitment to South Korea.REF

The United States could be in a high-intensity conflict with China at any point in the foreseeable future. Such a contest—whomever the victor—would change the power dynamics across East Asia and the Western Pacific. Should the United States win, China would be set back militarily and likely would see its geopolitical ambitions set back years, possibly decades. Should China defeat the United States, China would become almost assuredly the hegemon within the region—possibly on a path to become a global superpower; one that would rewrite the rules of the global order. Such an outcome would not be in the national interest of the United States—and certainly not South Korea.

The South Koreans have long been encapsulated by fears of North Korea. While there is great merit to such fears, given the experiences of the Korean War and the nuclear threats posed by North Korea, South Korea cannot disregard other security concerns in the Pacific theater.

South Korea’s Strengths

Indeed, given South Korea’s economic, political, industrial, and manpower, it can and should be a major regional power.

South Korea is the world’s 14th largest economy.REF It has one of the strongest defense industrial bases in the free world.REF It has a population of 51.7 million people.REF Indeed, if it chose to do so, it has the capability to be a stabilizing force across the greater Indo–Pacific theater.

The strength of the South Korean industrial base is evident through the production of advanced systems. The K2 Black Panther, introduced in 2009, is possibly the most advanced tank in the world, and partners in Europe (including Norway and Poland) have been rapidly purchasing it.REF The K9A2 self-propelled howitzer, expected to be fielded in 2027, can fire 10 rounds per minute with a 25-mile range.REF The KF-21 Fighting Hawk is South Korea’s Gen 4.5 supersonic fighter jet.REF Most of the technology applied to the KF-21 comes from South Korea.REF

Furthermore, South Korea has developed the largest shipbuilding industry in the free world based on compensated gross tonnage, coming second overall to China.REF However, South Korea surpasses Chinese shipbuilding when it comes to quality: Korean production is focused on high-value vessels and the development of new technology.REF

South Korea’s extensive ammunitions and systems sets its military in an advantaged position. The quantity it possesses is a powerful implication of its strength and made even more effective by the application of advanced technology.

This theme is echoed by American policymakers, who talk about the strength of Washington’s allies in Asia—and at the same time the dangers of them being intimidated by China. Speaking to Asian partners during a security summit in Singapore, Secretary of Defense Pete Hegseth stated, “We will not be pushed out of this critical region, and we will not let our allies be subordinated and intimidated.”REF

Would South Korea Choose China Over the U.S.?

However, there is some concern in Washington that the new administration in Seoul may be unwilling to play a broader role in the Indo–Pacific and may be too accommodating to China and/or North Korea. The new South Korean president, Lee Jae-myung, has not been in office long enough for observers to be sure of his foreign policy outlook. During the recent presidential campaign, however, he alluded that he would be more willing to seek accommodation with North Korea and China.REF Lee said, “There is no need for us to intentionally exclude or stand hostile against China or Russia.”REF Some suggest that Lee and the Democratic Party would seek a way to stay out of a conflict between the United States and China.REF

A war with China would trigger Article III of the U.S.–South Korea Mutual Defense Treaty. If South Korea were to choose neutrality, many in the United States would likely question the utility of the U.S.–South Korea alliance. An alliance that requires the United States to come to the aid of South Korea in a war with a nuclear-armed North Korea but does not require South Korea to come to the aid of the United States in a war with a nuclear-armed China would be a one-sided alliance, indeed.

To that end, Seoul should understand that if the United States were in a high-intensity conflict with a nuclear-armed China, and South Korea chose to remain completely neutral, there would likely not be a U.S.–South Korea alliance at the end of such a conflict—irrespective of the outcome.

An alliance is formed because of mutual overlapping interests. In the case of the United States and South Korea, this interest is Northeast Asian security. Americans support the long-standing alliance and want it to continue well into the future. Seoul should understand that China is the preeminent security consideration of the United States, even as Washington understands that North Korea is the preeminent security concern of South Korea. With this as the starting point, the two old allies can chart a path forward that works for both parties.

Robert Peters is Senior Research Fellow for Strategic Deterrence in the Douglas and Sarah Allison Center for National Security at The Heritage Foundation. Cameryn Jones is a member of the Young Leaders Program at The Heritage Foundation.