Seoul faces heavy responsibility in U.S. strategy

A senior U.S. Defense Department official recently stressed that the first island chain remains the core of America’s defense posture. The chain, which runs from Japan’s Kyushu through Okinawa, Taiwan and the Philippines, has long served as both China’s primary maritime defense line and Washington’s traditional containment perimeter. The official noted that U.S. forces stationed there must serve as more than a symbolic presence and called on allies such as South Korea to play a greater role in deterring China.

The timing of this message, just before the release of the new National Defense Strategy, shows that U.S. President Donald Trump’s second administration has firmly set its military focus on containing China, particularly preventing an invasion of Taiwan. In the past, isolationist groups in Washington had argued for pulling U.S. forces from South Korea and other forward positions back behind the second island chain, which stretches from Japan’s Honshu through Saipan, Guam and Indonesia. By reaffirming the importance of the first island chain, the United States has eased concerns that South Korea could once again be excluded from the defense line under a so-called new Acheson Line.

Even so, the burden now placed on Seoul is heavy. Washington’s call for stronger allied contributions and the prospect of adjustments in its Asian deployments effectively mean South Korea will be asked to assume greater responsibility for deterring North Korea while the United States focuses on China. Although large-scale reductions of U.S. Forces Korea are unlikely in the immediate future, their mission could increasingly shift toward a Taiwan contingency. That may change the composition and character of the garrison, with ground forces reorganized into multi-domain units capable of long-range operations and air forces restructured for extended missions.

When Washington publishes its new Nuclear Posture Review together with the Global Posture Review, which outlines worldwide U.S. force realignments, the future of the South Korea-U.S. alliance and the role of U.S. Forces Korea will come into sharper focus. For South Korea, already caught in the pressures of U.S.-China rivalry, America’s pivot to China presents an even more difficult test. The friction and conflicts this change may generate will be Seoul’s responsibility to manage, and the government’s success in doing so will be the true measure of its capability.