This dissertation offers a historical sociology of the making and unmaking of rule in a frontier borderland in Vietnam. By thinking through the social processes accompanying the spatial expansion and configuration of state power, I argue that military conquest was a necessary but insufficient step towards an emergent nationstate's lasting consolidation of territorial claims. Legitimating state claims to territory rested on mutually constitutive and spatialized processes of institutionalizing relations of rule; agrarian development and reciprocal exchange; constructing communities of nation and ethnicity; and social mobilization during, and for purposes of, warfare. Because the first Indochina war ended dramatically in Dien Bien Phu, the past of what is now a national frontier is remembered and forgotten in particular ways. In Vietnam, the site is celebrated as battle and setting for victory in 1954 of revolutionary ideals over foreign oppression. What is not remembered in such nationalist histories has equal significance for understanding the sometimes contradictory outcomes of state making. This dissertation analyzes human beings as complex, thinking persons with wayward agendas who did not always respond as intended. Grounded in archival research, the dissertation attends to the ways in which state claims to territory and legitimate rule are contested and how outcomes are contingent and non-linear. The narrative begins when colonial officers and mapmakers encountered the Sipsongchauthai confederation in the late 19th century, recognized a ruling Thai elite, incorporated them into French Tonkin, and, in so doing, remade the Black River borderlands into an imperial frontier. After World War II, France organized the Thai Federation, created splits among local elites, and unintentionally generated support for the Democratic Republic of Vietnam's nationalist alternative. Although cadres legitimized national rule by delegitimizing a neocolonial rival, Vietnam's emergent -Northwest[DOUBLE VERTICAL LINE] region reproduced Tonkin's borders and underlying political configuration while political elites reproduced the colonial category of -ethnicity[DOUBLE VERTICAL LINE] to regulate uncertain boundaries between community and territory, land and class. The unsteady incorporation of Black River peoples into Vietnam illustrates an evolving-and ongoing-negotiation between highlands and lowlands, center and frontier, state and society, nation and ethnicity.