In the early part of the nineteenth century, the Chenla Kingdom developed as a vital and decisive role in the Nguyen dynasty's southern border defense strategy in the southwestern region of Vietnam. A special defense relationship with the Chenla Kingdom was established and maintained by the Nguyen Kings from Gia Long, all of whom attempted to do so on the basis of the protectorate policy that Vietnam had established on this country during the course of the power interaction with Siam. Vietnam and Chenla defense relationship was viewed as an efficient way to maintain both the peace in Chan Lap and a balance in the Vietnam - Siam relationship. Vietnam is a key player in that relationship, engaging in a variety of initiatives to maintain a peaceful space in both Chan Lap and Vietnam. It is the purpose of this essay to shed light on the defense cooperation between Vietnam and the Chenla Kingdom during the Nguyen Dynasty under Gia Long (1807-1820).