Recent developments have increasingly called into question U.S. security guarantees to its allies. These new worries build on deeper difficulties between the United States and the Republic of South Korea (ROK) over what they need for their joint defenses against Pyongyang in the face of the evolving threat from the Democratic Peoples’ Republic of Korea (DPRK). These troubles encompass Washington’s plans to sharplyreduce the U.S. military presence in the ROK from 520,000 to perhaps 450,000 activeduty soldiers, the two countries’ diverging missile defense architectures in Asia, elevated ROK interest in acquiring preemptive strike options against the DPRK, and other defense and deterrence differences. Yet, the ROK-U.S. alliance has overcome earlier challenges of equal magnitude and its enduring goals remain unchanged: to help deter North Korean aggression, maintain South Koreans’ security and welfare, encourage the ROK to contribute to regional and global stability, and work toward Korean reunification. With skilled management, the ROK-U.S. partnership should be able to overcome its current troubles and pursue these goals more effectively, provided the alliance continues to evolve to respond to contemporary challenges and new realities.