Nationalism, national identity, and ethnicity are complex social phenomena worldwide and especially so in post-Soviet Ukraine. This monograph explores the causes and conditions of post-communist nationalist revivals focusing on the re-emergence of Cossack movements in Russia and Ukraine since the late 1980s. The study explores how different theories of nationalist movements underpinned different national policies and, ultimately, different socially constructed realities that led to the armed conflict between Russia and Ukraine.