AbstractBetween the February revolution and the 1921 end of the Russian Civil War, Buryat nationalists built a nation around Lake Baikal. Leaders sought Buryat autonomy within a postrevolutionary Russian polity. A lengthy border with Mongolia framed the region’s political geography and state‐builders competed for Buryat allegiances, compelling Buryat nationalists to make the nation legible to Russian authorities. Using Russian and Mongolian archival sources alongside published materials, this article makes three arguments. First, nationalists understood Buryats as Mongols and, thus, pursued Buryat ethnic consolidation. Second, Buryat members of the Cossack estate triggered political anxieties. Cossacks demanded their loyalty to the Transbaikal Cossack Host while nationalists hoped to remove them from the Cossack estate and bind them to the Buryat nation. Tensions between imperial and ethnic social categories, not alignment with parties characteristic of the revolutions and Civil War, sparked bloodshed in the Transbaikal. Finally, the region’s borderland orientation enabled and undermined the Buryat project. Many Buryat‐Cossacks challenged Buryat autonomy by embracing non‐national political affiliations in the Transbaikal and neighboring Mongolia. Ultimately, the Buryat‐Cossack question typified a concern of early‐twentieth‐century state‐builders: how would nations emerge in borderlands where legal hierarchies fragmented populations to forestall nationalism and affirm ruling dynasties’ supremacy?