This article introduces the ‘deterrence failure’ concept and applies it to North Korea’s nuclear threat situation. For this purpose, it selects five factors, including retaliation posture; credibility of retaliation; probability of the challenger’s success; challenger’s irrational leader; and situational desperateness, to evaluate the US–South Korea nuclear deterrence posture against North Korea. Except for the retaliation posture, most factors are very concerning. Therefore, to strengthen their deterrence posture against North Korea, the United States and South Korea should take a few practical measures to ensure the implementation of the US extended deterrence, including the forward-deployment of more US retaliation assets around the Korean Peninsula.