This seminar paper seeks to reconstruct how ethnic politics and inter-communal relations in Eastern Europe during World War I were central to the war’s conduct and legacy. Examining different popular and institutional understandings of ethnicity in the Russian and German Empires prior to 1914 and during the war proved to be key to understanding the breakdown of relations between occupying Germans, local nationalists, and Jewish communities, paving the way for the contentious ethnic politics of interwar Europe, which in-turn played a key role in driving Europe towards the destruction of World War II.