This article addresses the challenging spatial organization of Nguyễn Vietnam: the binary relationship between civilizational expansion and the construction of a state boundary at the Khmer frontier. It examines the process whereby the Vietnamese moved southwest into the Khmer world and territorialized a contested terrain as part of a civilizational and imperial project. The process employed the state’s administrative infrastructure and cultural institutions to erase ethnic, political, and cultural diversity in the lower Mekong. This article argues that Vietnamese expansion was not simply an attempt to carry out the will of heaven and Confucian cultural responsibility; rather, it was a search for peripheral security and a response to regional competition. In fact, the seesawing between civilizational mission and territorial consolidation confused the Nguyễn bureaucracy with regard to Cambodia’s political and cultural status and affected Hue’s frontier management. As a result, the Vietnam-Cambodia boundary was the object of frequent shifts and negotiations. Only after facing Siamese invasion and experiencing fierce Khmer resistance did the Vietnamese court gradually replace its civilizational perspective with a more practical approach to border management, out of which emerged the modern borderline.