As Germany fought the Soviet Union during World War II, a much smaller but equally vicious struggle was unfolding in southeastern Poland, fueled by longstanding ethnic and territorial conflicts between Poles and Ukrainians. Both sides organized large partisan armies and sought control over territory each deemed integral to their postwar national visions. The violence reached a fever pitch in the years immediately following the war. This comprehensive study surveys Polish–Ukrainian relations dating back to the tenth century. Rapawy follows centuries of ethnic strife, population shifts, and the formation of national states after the First World War on multi-ethnic territories, illuminating the long-term historical processes that informed later events.