Insurgency has two main strategies, guerrilla warfare and terrorism, which should be treated as linked, but distinct, strategies. This thesis examines the role of incumbent violence in leading insurgents to select one, or both, of these strategies. It argues that incumbent violence can create support for insurgency by causing fear and a desire for revenge and reshaping the social structures of a community. It also argues that incumbent violence increases popular support for terrorism in particular by creating outbidding incentives and desires to respond in kind to civilian deaths and as a way of punishing norm violations against attacking civilians on the part of the incumbent. The paper tests this theory with a qualitative case study of the conflict in Northern Ireland during the 1970s and a quantitative analysis of insurgent violence in the Kirkuk, Diyala, Babylon, and Salah al Din provinces during the 2003-2009 Iraq conflict