The period of the 1970s was a turning point in relations between North and South Korea. This was the time of the first peaceful negotiations between the two governments but also the time of one of the worst security crises since the Korean War. Despite the importance of examining this period to understand the dynamics of inter-Korean relations in general and of the effect of Sino-American rapprochement in particular, the relations between North and South Korea in the 1970s have rarely become an object of academic inquiry. The existing studies mostly cover only the period of the early 1970s and are limited in focus to either a particular actor(s) or to certain aspects of foreign relations of the two regimes that they investigate. This analysis demonstrates that the transformation from bilateral dialogue to crisis on the Korean Peninsula in the mid-1970s developed through the gradual escalation of tensions and was the consequence of both the diplomatic and the military policies pursued by the two regimes. As such, the security crisis was closely connected to the diplomatic competition that the two Koreas engaged in on a global scale, and, particularly, at the United Nations (UN), where the security and legitimacy interests of North and South Korea overlapped and collided. I suggest the concept of a “diplomatic war” to highlight the unique characteristics of inter-Korean relations in the early to mid-1970s—the intensity of the competition and the coordination and interrelatedness between the security and legitimacy agendas of the two regimes.