This study offers important clues to understanding the current dynamics of China’s relations with Vietnam and Russia. The Sino-Vietnamese conflict of the late 1970s represents a significant example of the contradictions between socialist states. It is important to emphasize that this conflict had deep historical antecedents, and its key catalyst, in addition to bilateral disagreements, was the already damaged Sino-Soviet relations. Thus, this study opens valuable perspectives for analyzing the evolution of Sino-Soviet (Russian) relations, the transformation of Sino-Vietnamese interactions, and the changing geopolitical landscape in Southeast Asia. This article examines how, after the unification of Vietnam, the national interests of China and Vietnam evolved from convergence to fundamental conflict, resulting in differences over practical interests surpassing traditional ideological ties. Key contradictions are analyzed: territorial disputes, the issue of the Chinese diaspora (Huaqiao), and the struggle for influence in Kampuchea. Particular attention is given to the role of the Sino-Soviet split, when Moscow, through military aid and a political alliance with Hanoi, contributed to the containment of China. The article concludes by revealing that Sino-Vietnamese cooperation was initially fragile and was driven by shared strategic imperatives in the period before Vietnam’s unification. After 1975, the geopolitical situation changed dramatically: a unified Vietnam began to pursue a more independent foreign policy, and the deepening Sino-Soviet split created the conditions for Hanoi’s reorientation. Under these circumstances, Vietnam chose a course of rapprochement with the Soviet Union, which led to the formation of a de facto anti-Chinese alliance and a final confrontation with China in the late 1970s.