Is the Trump administration's security policy drawing a new Acheson Line?
The contours of the Trump administration’s second-term security policy for Northeast Asia are becoming clearer. On Sunday, U.S. Secretary of Defense Pete Hegseth visited Japan and declared that the U. S. is “moving swiftly to rebuild deterrence.” He emphasized that “Japan is an essential partner in deterring China’s military aggression” and expressed his expectation that Japan would invest in its defense capabilities to secure a free future. With President Donald Trump and other senior U.S. officials referring to North Korea as a “nuclear power” while prioritizing efforts to contain China, Washington’s strategic focus is evident.
![US Secretary of Defense Pete Hegseth speaks during a meeting with British Secretary of State for Defence John Healey at the Pentagon in Washington, DC, on March 6, 2025. US defense chief Pete Hegseth will meet his Philippine counterpart in Manila on March 28, part of a tour of Pacific allies that risks being overshadowed by a mounting scandal over leaked plans for military strikes.[AFP/YONHAP]](https://koreajoongangdaily.joins.com/data/photo/2025/04/01/70f5ec27-1ef2-484e-a33c-eab16400e083.jpg)
A classified document signed by Hegseth, titled the “Interim National Defense Strategic Guidance,” reinforces this stance by identifying China as the sole major threat. According to a leaked nine-page version published by U.S. media, Washington has decided to prioritize defending its homeland and Taiwan, even at the risk of exposure elsewhere. In contrast, U.S. allies such as South Korea and Japan — who face North Korea’s nuclear and conventional threats — are expected to take responsibility for their own defense. This shift in policy could destabilize the security order in Northeast Asia. Given that Hegseth chose Japan, a key pillar of trilateral security cooperation with South Korea and the U.S., to articulate this position, it is likely that Washington will soon make similar demands of Seoul.
This development is particularly unsettling for South Korea, which has been bracing for Trump’s demands for a drastic increase in defense cost-sharing. Moreover, it risks sending the wrong message to North Korea. In 1950, the United States excluded South Korea from its defense perimeter — known as the Acheson Line — a move some analysts believe emboldened North Korea’s invasion of the South. The Trump administration’s evolving security doctrine raises concerns about the emergence of a second Acheson Line.
The South Korean government must take all necessary steps to ensure that Washington’s new stance does not weaken the Korea-U.S. combined defense posture. Some analysts warn that the U.S. may attempt to repurpose Camp Humphreys, the U.S. military base in Pyeongtaek, or redeploy U.S. forces stationed in Korea to counter China. In response to growing concerns, Defense Ministry spokesperson Jeon Ha-kyu reassured the public on Monday, stating that “the primary role of U.S. Forces Korea is to maintain peace and stability on the Korean Peninsula, and that remains unchanged.” However, Washington’s recent actions suggest otherwise: while the U.S. National Defense Authorization Act stipulates that U.S. troop levels in Korea should not fall below 28,500, the law no longer includes a provision prohibiting the use of authorized funds for troop reductions. Depending on developments in Taiwan, the U.S. may seek to redeploy its forces or expedite the transfer of wartime operational control (OPCON) to South Korea.
![US Secretary of Defense Pete Hegseth (centre, L) shakes hands with Japan's Prime Minister Shigeru Ishiba during their meeting at the Prime Minister's office in Tokyo on March 30, 2025. The United States will ensure ″robust, ready and credible deterrence″ across the Taiwan Strait, US Defense Secretary Pete Hegseth said on March 30, calling China ″aggressive and coercive″.[AFP/YONHAP]](https://koreajoongangdaily.joins.com/data/photo/2025/04/01/2e308ee7-7517-4f36-b5c7-6acef89eff7b.jpg)
Now more than ever, Seoul must strengthen its cooperation with Washington and proactively monitor U.S. policy shifts. The government must not repeat past missteps — such as belatedly confirming South Korea’s designation as a “sensitive country” by the U.S. only after the media reported it. To navigate the unpredictable waves of Trump-era security policy, South Korea must be fully prepared with its own strategic leverage and contingency plans.
Translated from the JoongAng Ilbo using generative AI and edited by Korea JoongAng Daily staff.
US Secretary of Defense Pete Hegseth speaks during a meeting with British Secretary of State for Defence John Healey at the Pentagon in Washington, DC, on March 6, 2025. US defense chief Pete Hegseth will meet his Philippine counterpart in Manila on March 28, part of a tour of Pacific allies that risks being overshadowed by a mounting scandal over leaked plans for military strikes.[AFP/YONHAP]
A classified document signed by Hegseth, titled the “Interim National Defense Strategic Guidance,” reinforces this stance by identifying China as the sole major threat. According to a leaked nine-page version published by U.S. media, Washington has decided to prioritize defending its homeland and Taiwan, even at the risk of exposure elsewhere. In contrast, U.S. allies such as South Korea and Japan — who face North Korea’s nuclear and conventional threats — are expected to take responsibility for their own defense. This shift in policy could destabilize the security order in Northeast Asia. Given that Hegseth chose Japan, a key pillar of trilateral security cooperation with South Korea and the U.S., to articulate this position, it is likely that Washington will soon make similar demands of Seoul.
This development is particularly unsettling for South Korea, which has been bracing for Trump’s demands for a drastic increase in defense cost-sharing. Moreover, it risks sending the wrong message to North Korea. In 1950, the United States excluded South Korea from its defense perimeter — known as the Acheson Line — a move some analysts believe emboldened North Korea’s invasion of the South. The Trump administration’s evolving security doctrine raises concerns about the emergence of a second Acheson Line.
The South Korean government must take all necessary steps to ensure that Washington’s new stance does not weaken the Korea-U.S. combined defense posture. Some analysts warn that the U.S. may attempt to repurpose Camp Humphreys, the U.S. military base in Pyeongtaek, or redeploy U.S. forces stationed in Korea to counter China. In response to growing concerns, Defense Ministry spokesperson Jeon Ha-kyu reassured the public on Monday, stating that “the primary role of U.S. Forces Korea is to maintain peace and stability on the Korean Peninsula, and that remains unchanged.” However, Washington’s recent actions suggest otherwise: while the U.S. National Defense Authorization Act stipulates that U.S. troop levels in Korea should not fall below 28,500, the law no longer includes a provision prohibiting the use of authorized funds for troop reductions. Depending on developments in Taiwan, the U.S. may seek to redeploy its forces or expedite the transfer of wartime operational control (OPCON) to South Korea.
US Secretary of Defense Pete Hegseth (centre, L) shakes hands with Japan's Prime Minister Shigeru Ishiba during their meeting at the Prime Minister's office in Tokyo on March 30, 2025. The United States will ensure ″robust, ready and credible deterrence″ across the Taiwan Strait, US Defense Secretary Pete Hegseth said on March 30, calling China ″aggressive and coercive″.[AFP/YONHAP]
Now more than ever, Seoul must strengthen its cooperation with Washington and proactively monitor U.S. policy shifts. The government must not repeat past missteps — such as belatedly confirming South Korea’s designation as a “sensitive country” by the U.S. only after the media reported it. To navigate the unpredictable waves of Trump-era security policy, South Korea must be fully prepared with its own strategic leverage and contingency plans.
Translated from the JoongAng Ilbo using generative AI and edited by Korea JoongAng Daily staff.
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