Extended nuclear deterrence has been a cornerstone of U.S. defense and foreign policy and a foundation of ROK-U.S. security ties. This paper analyzes the changing dynamics of U.S. extended nuclear deterrence on the Korean peninsula since the eruption of the North Korean nuclear crisis. It focuses on the negative security assurance (NSA) and explains how U.S. extended nuclear deterrence for South Korea is negatively affected by the new NSA of the Obama Administration. It also investigates the history of security guarantees to North Korea provided by the United States and emphasizes how America’s repeated security guarantees—in its attempt to denuclearize North Korea—have weakened the nuclear umbrella over South Korea. In addition, the paper points out that some policies of the United States at the negotiations to resolve the North Korean nuclear crisis can only be explained by American politicians’ drive for a positive political legacy. Therefore, some safeguard measures are necessary to supplement the diminishing American nuclear umbrella on the Korean peninsula. The unique threat to South Korea makes it a suitable place to actively apply the new concept of regionally tailored deterrence architecture and implement a dual-track approach to negotiations for resolving the North Korean nuclear crisis by preparing to redeploy U.S. tactical nuclear weapons to South Korea. Finally, the paper explicates South Korea’s non-nuclear weapon policy and proposes a new concept of the South Korean alliance security assurance.