Departing from conventional studies of border hostility in inter-Asian relations, Yin Qingfei explores how two revolutionary states – China and Vietnam – each pursued policies that echoed the other and collaborated in extending their authority to the borderlands from 1949 to 1975. Making use of central and local archival sources in both Chinese and Vietnamese, she reveals how the people living on the border responded to such unprecedentedly aggressive state building and especially how they appropriated the language of socialist brotherhood to negotiate with authorities. During the continuous Indochina wars, state expansion thus did not unfold on these postcolonial borderlands in a coherent or linear manner. Weaving together international, national, and transnational-local histories, this deeply researched and original study presents a new approach to the highly volatile Sino-Vietnamese relations during the Cold War, centering on the two modernising revolutionary powers' competitive and collaborative state building on the borderlands and local responses to it.