In 1802, the Nguyen Dynasty in Vietnam was established and grew up strong. In that context, in Siam (Thailand), the Rama Dynasty is also growing strong and wants to have a leading position in mainland Southeast Asia. Therefore, there was a war between Vietnam and Siam (the first war 1833-1834, the second war 1841-1845). The war between Vietnam and Siam in the nineteenth century was the successor to the previous wars between the two countries in the past, but in the nineteenth century it was implemented at a higher and more drastic level. This war caused a lot of losses for both countries but more importantly the Vietnam - Siam war set in the geopolitical context of mainland Southeast Asia in the nineteenth century showed fierce competition between two countries to control Cambodia and Laos. This war has left behind historical issues worthy of research attention, showing very difficult diplomatic relations between Vietnam and Siam. It also shows the complex international relations in Southeast Asia in the nineteenth century and this partly explains the current Vietnam-Thailand relations as well as provides an important historical content of international relations in Southeast Asia.

The study uses military history research methods, international relations research methods, and actual survey methods to clarify the research problem. Thereby contributing to clarifying an important historical issue of Southeast Asian history in the nineteenth century.