The second Trump administration, advancing an "America First" posture, has adopted a transactional approach that no longer provides unconditional security guarantees to allies but instead demands commensurate contributions in return. Within this framework, the United States has designated the deterrence of a Chinese invasion of Taiwan as the foremost priority of its defense strategy, and voices have been raised arguing that South Korea should bear primary responsibility for deterring North Korea while the United States intervenes only in the event of Chinese involvement. This reasoning leads to a logic of shifting the role of United States Forces Korea (USFK) from deterrence of North Korea to counterbalancing China, presenting South Korea with the contentious tasks of accommodating expanded strategic flexibility for USFK and contributing to regional security.

South Korea, however, has had little choice but to approach the Taiwan question with caution, given the imminent threat posed by North Korea and the fact that South Korea-China relations are oriented toward a strategic cooperative partnership. Nevertheless, China's military buildup, the expansion of its military activities in the vicinity of the Korean Peninsula, and the pursuit of "alliance modernization" under the second Trump administration demonstrate that South Korea can no longer afford to look away from these issues. Accordingly, voices within South Korea have called for maintaining the primacy of the North Korean threat while partially accommodating United States demands and proactively advancing the modernization of the ROK-U.S. alliance in order to prepare for potential threats and preserve the continuity and credibility of the alliance.

Against this backdrop, the present study seeks to reexamine the scope of United States demands and South Korean accommodation through the concept of "bidirectional extended deterrence." It examines whether a structure in which the United States and South Korea mutually and symmetrically guarantee and cooperate on deterrence is feasible, as a departure from the asymmetric deterrence structure dependent on the United States nuclear umbrella. To this end, the study employs a 4C analytical framework comprising Communication, Capabilities, Credibility, and Continuity to diagnose the uncertainties of ROK-U.S. extended deterrence under the second Trump administration, and sets forth the conditions necessary for sustaining "integrated extended deterrence" and restoring alliance credibility.

In conclusion, bidirectional extended deterrence appears to be an inevitable challenge that the future of the ROK-U.S. alliance will have to confront. While full accommodation of United States demands is not feasible, a phased and conditional bidirectional transition that does not explicitly target third countries could instead serve as an opportunity to enhance the institutional durability and credibility of the alliance. Should South Korea avoid making contributions, the alliance will be destabilized through defense cost pressures and adjustments to USFK, while conversely, excessive contributions could weaken deterrence against North Korea and provoke a Chinese backlash. South Korea must construct a balanced model that maintains integrated extended deterrence cooperation centered on deterrence of North Korea while gradually contributing to extraregional stability.​