In the early of the 19^th century, Vietnam and Siam played a strong dominant role on the Indochina peninsula. Vietnam is constantly expanding its influence to the West to create a security-space for the Southern land. Siam is also continually enlarged its influence to the East to annex land and increase its influence abroad. Meanwhile, Chenla is located between two powerful forces, so it is increasingly threatened and dependent. Between these three countries, there exist complex types of power interactions, affecting the existence and development of each country. The article focuses on analyzing the triangular relationship between Vietnam - Chenla - Siam approach from the foreign policy of King Gia Long (1802-1820) to clarify efforts to protect and maintain peaceful space in the border areas in the Southwest of Vietnam faced the threat from Siam during this period.