One of the greatest threats to state security in the post Cold War era has been the rise of nationalism and the role of ethnicity in political mobilization. We discuss the roots of ethnic and nationalistic sentiment, specifically whether ethnicity is a flexible social construct. We then move forward to consider the following questions. When does ethnicity become politicized? How and why do ethnic groups mobilize to engage the state? What are possible state responses to nationalist sentiment and ethnic conflict? Throughout the class case studies of recent ethnic conflict, such as Yugoslavia and the Rwandan genocide, will be used to illuminate the material.