This study addresses the question of why the South Korean government sought to join the Southeast Asia Treaty Organization (SEATO) in the late 1960s, despite the existence of the South Korea-U.S. alliance. The research identifies South Korea's consideration of SEATO membership as a response to a series of security threats faced by the country after 1968, such as the January 21 Blue House raid, the capture of the USS Pueblo, and the implications of the Nixon Doctrine. In response to these threats, the South Korean government sought to reinforce the U.S. security commitment to South Korea by attempting to revise the ROK-U.S. Mutual Defense Treaty and to establish new multilateral security organizations in Asia, such as the Asia-Pacific Treaty Organization (APATO). However, these efforts faltered, leading the South Korean government to consider SEATO membership as a last resort to strengthening its security through alliances. Despite SEATO not providing a qualitative improvement in U.S. security commitments, membership could have functioned as a dual safety net alongside the ROK-U.S. Mutual Defense Treaty. However, South Korea's attempt to join SEATO was based on the strategic miscalculation that SEATO would persist and strengthen, and it did not foresee the U.S. opposition to Korean membership. From the U.S. perspective, any expansion of security commitments in Asia was untenable, and South Korea's SEATO membership was seen as an unacceptable policy extension. In conclusion, the failure to join SEATO marked a turning point where South Korea's security policy shifted from prioritizing alliances to prioritizing self-reliance, leading to the full-fledged emergence of the self-defense policy in the 1970s.
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