This article analyzes territorial disputes and political relationships at the border between China and Vietnam from the seventeenth to the nineteenth centuries. Predominant Western scholarship argues that, owing to the tributary relationship among states and polities, there was no territorial boundary in premodern Asia; furthermore, it suggests, the concept of the “geo-body” of a nation or sovereign state only arose with the transfer of new mapping technology from Europe. This article argues instead that the absence of lines of demarcation on Vietnamese and Chinese maps before the late nineteenth century does not connote a lack of consciousness of the existence of borders. The quest for autonomy throughout history by local communities living between China and Vietnam gave rise to border conflicts, which led to the intervention by and expansion of these two states, as well as negotiations and territorial division between them. The transformation of the China-Vietnam border from a premodern to a modern form thus did not depend solely on its cartographic representation; it also involved the power of the state to control space. Additionally, this article demonstrates that tensions over the border did not simply involve central governments but often resulted from a combination of local conflicts and the complicated relations between local actors and the state. The article suggests a new approach to exploring the history of state borders from the perspective of local people, in which the “in-between communities” are not seen as passive objects of border demarcation but are also a driving force in the establishment of a frontier. While the “in-between communities” discussed in this article were behind conflicts over land and its division into national territories, their manipulations of ethnic identity and transgressive mobility also helped blur the border between the two countries.