March 15, 2023

Peninsula Plus: Enhancing U.S.–South Korea Alliance Cooperation on China, Multilateralism, and Military and Security Technologies

Executive Summary

The United States–Republic of Korea (ROK or South Korea) alliance has entered a critical phase. In 2023, the two countries will commemorate the 70th anniversary of signing their bilateral mutual defense treaty. This year also marks the first full year under national leaders President Joe Biden and President Yoon Suk Yeol. After several challenging years in the two countries’ relationship, ties are improving. Better alliance relations have, unfortunately, coincided with a deterioration in the regional and global security environment, specifically due to threats from North Korea, China, and Russia. This report examines the U.S.-ROK alliance as it adapts to the new regional context by exploring how the United States and South Korea can sustain and deepen their relationship in three vital policy areas: coordination on China, alignment in minilateral and multilateral settings, and defense technology collaboration.

Perhaps the biggest shift in alliance priorities in recent years has been the growing importance of the China challenge. During Chinese Communist Party General Secretary Xi Jinping’s decade in power, Beijing has adopted a more muscular foreign policy. Both the United States and South Korea have reshaped their approaches toward China in response. ROK concerns about China have grown as Beijing shields Pyongyang and acts aggressively elsewhere in the region, including toward Taiwan. But South Korea’s approach to China will continue to differ from that of the United States. The allies have divergent preferences regarding the speed, manner, and degree of partial decoupling with China. Moreover, South Korea’s deep trade ties with China will continue to make it vulnerable to Chinese political and economic coercion.

Perhaps the biggest shift in alliance priorities in recent years has been the growing importance of the China challenge.

More broadly across the region, the 2022 Indo-Pacific Strategies of the Biden and Yoon administrations both place strong emphasis on multilateralism and more robust engagement with the region. The United States and South Korea have many opportunities to expand their cooperation beyond the Korean Peninsula, including through the U.S.-ROK-Japan trilateral relationship, engagement with the Quad (the United States, Australia, India, and Japan), deeper involvement with NATO and Europe, and other minilateral and multilateral relationships. Efforts along these lines are integral to the Yoon administration’s vision of transforming South Korea into a “global pivotal state.”

As South Korea prepares for a larger role on the international stage, it must also continue to keep pace with the evolution of North Korean and Chinese military capabilities. Over the past several decades, the South Korean defense industry has transformed from near-complete dependency on foreign hardware to becoming one of the world’s top defense suppliers. The end of U.S.-imposed missile and weapons restrictions on South Korea in 2021 supercharged a series of defense technology modernization initiatives during former President Moon Jae-in’s administration, which have continued under Yoon. Given Seoul’s ambition to build an independent domestic defense industry, the opportunities for innovations driven by American and Korean collaboration on the scientific cutting edge, along with greater defense technology integration, are substantial.

In consideration of the above challenges and opportunities, the report recommends that U.S. and ROK alliance policymakers:

  • Identify and build on overlapping priorities in the U.S. and ROK Indo-Pacific Strategies;
  • Broaden the alliance to account for threats from China in addition to North Korea;
  • Encourage Seoul’s ambitions beyond the Korean Peninsula while also acknowledging the concomitant pressures of a larger global role—especially from Beijing;
  • Take a bold approach to boosting U.S.-ROK-Japan trilateral cooperation;
  • Facilitate South Korea’s deeper integration into minilateral coalitions with leading democracies;
  • Create a defense technology development and acquisition roadmap for the alliance that strengthens both deterrence and stability;
  • Deepen U.S.-ROK cooperation in the cyber and space domains;
  • Negotiate a U.S.-ROK reciprocal defense procurement agreement; and
  • Jointly reexamine from first principles all options for bolstering allied nuclear deterrence, but be realistic about the negative consequences of drastic changes in capabilities and posture.


The United States–Republic of Korea (ROK or South Korea) alliance has entered a critical phase. The two countries celebrated 140 years of diplomatic ties in 2022 and will commemorate the 70th anniversary of signing their bilateral mutual defense treaty in 2023. This year also marks the first full year under President Joe Biden’s administration in Washington and President Yoon Suk Yeol’s administration in Seoul. It further signals the final transition from an earlier period of relations under U.S. President Donald Trump and ROK President Moon Jae-in during which bilateral rifts and geopolitical turbulence shook the foundations of the alliance to a degree not seen in decades. During the Trump-Moon years, both allies weathered the journey through a stretch of escalating military brinksmanship characterized by “fire and fury” rhetoric that threatened to engulf the Korean Peninsula in conflict. Next came a stint of frenetic diplomacy between Washington and Pyongyang that saw some breakthroughs but ultimately ended in failure. That was quickly followed by the upheaval of a global pandemic.

The alliance’s weathering of that period and overall resilience reflect several sources of strength. One is the partnership’s deep history going back to the Korean War. Another source is the vast connections between the two societies at both the elite and popular levels. The alliance also benefits from extensive institutional links as well as the fact that Washington and Seoul are both liberal democracies with market economies. The greatest source of strength is their enduring mutual strategic interests in countering the threat from the Democratic People’s Republic of Korea (DPRK or North Korea) and, increasingly, from the People’s Republic of China (PRC or China) and the Russian Federation (Russia).

Recognizing those strengths as well as the challenges the U.S.-ROK alliance had faced in recent years, the Biden administration came into office seeking to enhance alliances and partnerships—including with South Korea—as the focal point of its foreign policy strategy, both in the Indo-Pacific and globally. Translating that agenda into action meant prioritizing early leader-level meetings. Moon was the second world leader to visit the White House in early 2021 shortly after President Biden took office. Biden then met with President Yoon in May 2022, marking the earliest U.S.-ROK leaders’ summit of any South Korean president’s term.

The greatest source of strength is their enduring mutual strategic interests in countering the threat from North Korea and, increasingly, from China and Russia.

Those meetings resulted in several outcomes, including a Special Measures Agreement on alliance burden-sharing, termination of the Revised Missile Guidelines that previously limited the range of South Korean missiles, a slate of ROK investments into the United States, and agreements between the leaders to embark on a “new chapter” and upgrade the relationship to a “global comprehensive strategic alliance.” The momentum extended to the release of key strategic documents by each country, including the U.S. National Security, National Defense, and Indo-Pacific Strategies and the ROK Indo-Pacific Strategy. Those documents all expressed support for a peaceful, stable, and rules-based order in the region.

The improvement in U.S.-ROK alliance relations has coincided with a deterioration in the regional and global security environment. The Kim Jong Un regime in North Korea continues its provocative behavior by advancing its nuclear and conventional missile capabilities, adopting a law enshrining a new and aggressive nuclear employment doctrine, and, at the time of writing, preparing to conduct a seventh nuclear test. While Chinese Communist Party (CCP) General Secretary Xi Jinping has made some diplomatic overtures toward other countries after he secured a third five-year term at the 20th National Congress of the CCP last October, Beijing has continued aggressive military patrols around Taiwan. Beijing’s diplomatic moves likely represent a tactical recalibration rather than a strategic shift of China’s foreign policy. Xi’s ambitious regional and global objectives and willingness to employ economic, political, and military coercion to achieve them persist. Meanwhile, Russia’s brutal war against Ukraine has demonstrated that the threat of large-scale conventional military aggression under the nuclear shadow is real. Moscow’s war in Ukraine has raised alarms globally about the potential for similar aggression by China in the Indo- Pacific, especially in light of the deepening China-Russia strategic partnership.

This report examines the future of the U.S.-ROK alliance in a dynamic moment marked by enormous security challenges as well as some new cooperation opportunities. While the alliance faces multiple challenges—not least, deterring North Korea—this report focuses on three areas that have been relatively understudied by policy and academic communities. First, it explores how the allies are responding individually and jointly to challenges posed by China. Second, it maps the agenda for alliance cooperation in minilateral and multilateral forums, particularly as Seoul looks to become what Yoon has called a “global pivotal state.” And third, the paper assesses the potential for joint efforts related to technology with military and security applications, building on earlier CNAS work on cooperation on civilian technologies. It concludes with recommendations for U.S. and ROK policymakers.

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  1. U.S. Department of State, U.S. Relations with the Republic of Korea – Bilateral Relations Fact Sheet (September 22, 2020),
  2. The White House, Indo-Pacific Strategy of the United States (February 2022),; and The White House, National Security Strategy (October 2022),
  3. The White House, Office of the Press Secretary, “Remarks by President Biden and President Moon Jae-in of the Republic of Korea Before Bilateral Meeting,” State Dining Room, Washington, May 21, 2021,
  4. “United States – Republic of Korea Leaders’ Joint Statement,” The White House, press release, May 21, 2022,
  5. “U.S.-ROK Leaders’ Joint Statement,” The White House, press release, May 21, 2021,; and “United States – Republic of Korea Leaders’ Joint Statement.”
  6. The White House, National Security Strategy; U.S. Department of Defense, National Defense Strategy of the United States of America (October 2022),; and The White House, Indo-Pacific Strategy of the United States; and The Government of the Republic of Korea, Strategy for a Free, Peaceful, and Prosperous Indo-Pacific Region (December 2022),
  7. Choe Sang-hun, “North Korea Adopts New Law Hardening Its Nuclear Doctrine,” The New York Times, September 9, 2022,
  8. Jacob Stokes, Alexander Sullivan, and Joshua Fitt, “Digital Allies: Deepening U.S.–South Korea Cooperation on Technology and Innovation” (Center for a New American Security, March 22, 2022),


  • Jacob Stokes

    Senior Fellow, Indo-Pacific Security Program

    Jacob Stokes is a Senior Fellow for the Indo-Pacific Security Program at CNAS, where his work focuses on U.S.-China relations, Chinese foreign and military policy, East Asian ...

  • Joshua Fitt

    Former Associate Fellow, Indo-Pacific Security Program

    Joshua Fitt is a former Associate Fellow for the Indo-Pacific Security Program at CNAS. He focuses on U.S. East Asian security strategy and specializes in Japanese and Korean ...

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