In late 2023, North Korea announced its intention to attack and annex South Korea despite the U.S. nuclear umbrella protecting South Korea. Simultaneously, South Korea has been threatening a decapitation operation against the North Korean leader. How does a small nuclear state like North Korea ignore the nuclear umbrella of the United States, a nuclear superpower, and how does a nonnuclear South Korea challenge North Korea? This chapter introduces the “balance of fear” (BOF) concept to search for answers to these two questions. It argues that both the conventional “balance of terror” concept and the “balance of resolve” concept cannot provide a sufficient answer. In contrast, the BOF concept explains that it is not the strength of nuclear forces, but the amount of fear that decides the failure or success in deterrence: North Korean leaders may have less fear of U.S. massive retaliation than U.S. leaders’ fear of North Korea’s suicidal nuclear attack on U.S. cities; South Korea’s decapitation operation may work because North Korean leaders’ fear of losing their lives could outweigh their potential achievement through a nuclear attack on South Korea.