Contributor(s): Professor Donatella Della Porta | From Gezi Park in Istanbul to Tahrir Square in Cairo, as well as in the heart of Europe, threatened regimes have faced down massive protests with brutal repression. But when do mass social movements go underground and choose violence? Della Porta brings to bear her extensive research into left-wing, right-wing, ethnonationalist, and religious forms of political violence to answer this question. The comparison of quite different cases of escalation allows to single out the competitive dynamics of radicalization both inside social movements and between them and the state. Donatella Della Porta is professor of political science and political sociology at the European University Institute with a distinguished record of research into social movements and political violence. Omar McDoom is assistant professor in comparative politics in the Department of Government at LSE.