The problem, at present, is not that the American commitment to fight a major war on behalf of an ally has weakened. Rather, it is that U.S. allies like South Korea have become subject to lower-level provocations that are very difficult to deter indeed. The U.S. nuclear umbrella is not necessarily designed to forestall nuclear weapons testing, or to dissuade the sinking of a frigate or the shelling of offshore islands. Yet when these acts occur, South Koreans feel fundamentally insecure.
This problem is not exclusive to the U.S.-ROK alliance. The recent Sino-Japanese standoff over the Senkaku/Diaoyu Islands has also raised the question of how extended deterrence functions at lower levels of escalation. It remains highly unlikely that the United States will ever have to decide whether or not to use nuclear weapons on South Korea’s behalf. China’s rise and North Korea’s continued nuclear and missile developments make it quite probable, however, that lower-level incidents will continue to surface, provoking anxieties from allies who feel that the U.S. umbrella is not water-tight. The return of tactical weapons may make some South Koreans feel that American nuclear use on their behalf is modestly more likely, but it will not solve the puzzle of how to shore up deterrence at lower levels of conflict.
South Korean calls for the return of NSNWs should not be dismissed, but taken as a signal that further assurance from the United States is needed. Whether through the recently established U.S.-ROK Extended Deterrence Policy Committee or other channels, the alliance must make coordination around lower-level threats a top priority. A failure to do so could result in serious alliance divisions when crises do arise, and only exacerbate feelings of insecurity in Seoul. Extended deterrence doubts could also have regional reverberations if they result in a more robust South Korean nuclear program.