How do separatist conflicts arise and spread? When does separatism become a cover for a foreign aggression? How do local communities respond when state institutions collapse, and militants take over? The armed conflict in Eastern Ukraine, which started eight years before Russia's full-scale invasion, contains unique evidence to address each of these questions. In Seize the City, Undo the State, Serhiy Kudelia offers an authoritative study of the conflict at its initial stage--2013-14--based on a meticulous comparison of mobilization dynamics in over dozen towns of Donbas as well as in two major cities outside of it: Kharkiv and Odesa. Through his extensive travels and numerous interviews with conflict witnesses and participants, Kudelia explains how a small group of Russian agents and local militants succeeded in eliminating state control over the largest and most densely urbanized region of Ukraine but failed to do it elsewhere. Kudelia challenges the conventional accounts of the armed conflict in Donbas, which portray it either as an interstate conflict entirely manufactured by Moscow or as a civil war that broke out without any external influence. Instead, he argues that local actors prepared ideological and organizational basis for the uprising, but the successful spread of separatist control resulted from the covert intervention of Russian agents and widespread collaboration with them of town administrators and community activists. His findings also show that when enough members of local communities organized to resist militant takeovers, the separatist challenges there quickly dissipated. A fine-grained and highly original on-the-ground analysis of the origins of the wider Russian-Ukrainian war that broke out in 2022, this book offers broader insights into the conditions under which external intervention may trigger the rise of an armed insurgency in a society torn apart by political and ideological disagreements.