Does unity within countries increase conflict between them? In this dissertation, I investigate whether it does using natural experiments. The first essay tests how exogenous surges of nationalism affect state aggression by exploiting a natural experiment that was created by the World Cup. The second essay analyzes whether united government increases state aggression by looking at a similar natural experiment. The third essay considers some of the methodological challenges that arose over the course of this project and draws lessons for future research. Along with shedding new light on the relationship between domestic unity and interstate conflict, this project demonstrates the value of using natural experiments to study international security.