This chapter provides an overview of the history of the Law of the Sea (LOS) with an emphasis on a series of international attempts to codify the LOS in the twentieth century. It explores the question of how participation in the negotiation process of the third UN Conference on the Law of the Sea (UNCLOS III) and the maritime regime in general affected China's conceptual and practical positions in respect to the South China Sea dispute. The chapter looks at the military confrontation between China and Vietnam that occurred in this period and identifies the factors shaping the decision-making of central leaders in Beijing. It analyses the concrete ways through which the maritime regime exerted its influence in the UNCLOS III period, independently and jointly with geopolitical flux. China joined the UNCLOS III immediately after its entry into the UN. Its maiden speech was made on March 3, 1972, only four months after its UN debut.