We explore how the dynamics of regime-changing insurgencies are influenced by a government’s legitimacy–that is, the popular acceptance of the governing regime as appropriate, proper, and just. Based on the theories of political and social science as well as case studies of insurgencies, we model legitimacy as a stock. Government actions—as they are perceived by the populace—can increase or decrease this stock. For example, the government can “accumulate” legitimacy by building roads, flood control systems, or other infrastructure. It can also create and maintain educational infrastructure that can, with long delays, shape the very categories of popular thought so that the people interpret government actions in a positive manner. In contrast, insurgent organizations use violence and propaganda to seek to decrease the stock of government legitimacy. For example, insurgent violence not only physically destroys government forces and infrastructure but also demonstrates that the government cannot guarantee security of the people, thus undermining confidence in the government. We simulate the model to provide a check on the internal consistency of the proposed relationships as well as their consistency with historically observed dynamic behavior. Through this research, we hope to integrate historical insights with organizational studies of legitimacy and extend them by explicitly including the effects of delays and nonlinearities in order to illuminate the points of leverage for addressing insurgencies.