During the past two centuries, western nations have successively extended the voting franchise to citizens of lower income. We explain this process of democratization as a rational way for incumbent elites to wage war effectively on other nations, as in a strategic game of international conflict handing over military spending decisions to citizens who face a lower tax cost of arming may confer a strategic delegation advantage. We find supporting empirical evidence in case studies of franchise extensions in the United Kingdom, France, and the United States.