The authors draw on a natural experiment to demonstrate that states can reconstruct conflictual interethnic relationships into cooperative relationships in relatively short periods of time. The article examines differences in how the gentile population in each of two neighboring territories in Romania treated its Jewish population during the Holocaust. These territories had been part of tsarist Russia and subject to state-sponsored anti-Semitism until 1917. During the interwar period one territory became part of Romania, which continued anti-Semitic policies, and the other became part of the Soviet Union, which pursued an inclusive nationality policy, fighting against inherited anti-Semitism and working to integrate its Jews. Both territories were then reunited under Romanian administration during World War II, when Romania began to destroy its Jewish population. The authors demonstrate that, despite a uniform Romanian state presence during the Holocaust that encouraged gentiles to victimize Jews, the civilian population in the area that had been part of the Soviet Union was less likely to harm and more likely to aid Jews as compared with the region that had been part of Romania. Their evidence suggests that the state construction of interethnic relationships can become internalized by civilians and outlive the life of the state itself.