Why do great powers engage in territorial expansion? Much of the existing literature views expansion as a largely intentional activity directed by the leaders of powerful states. Yet nearly 25 percent of important historical instances of great power expansion are initiated by actors on the periphery of the state or empire without authorization from their superiors at the center. Periphery-driven “inadvertent expansion” is most likely to occur when leaders in the capital have limited control over their agents on the periphery. Through their actions, peripheral agents effectively constrain leaders from withdrawing from these newly captured territories because of sunk costs, domestic political pressure, and national honor. When leaders in the capital expect geopolitical consequences from regional or other great powers, such as economic sanctions, militarized crises, or war, they are far less likely to authorize the territorial claims. A mixed-methods research strategy combines new quantitative data on great power territorial expansion with three qualitative case studies of successful (and failed) inadvertent expansion by Russia, Japan, and France. Inadvertent expansion has not completely gone away, particularly among smaller states, where government authority can be weak, control over states’ apparatuses can be loose, and civil-military relations can be challenging.