Winner of the International Security Studies Section Book Award of the International Studies AssociationIn Inadvertent Expansion, Nicholas D. Anderson investigates a surprisingly common yet overlooked phenomenon in the history of great power politics: territorial expansion that was neither intended nor initially authorized by state leaders. Territorial expansion is typically understood as a centrally driven and often strategic activity. But as Anderson shows, nearly a quarter of great power coercive territorial acquisitions since the nineteenth century have in fact been instances of what he calls "inadvertent expansion." A two-step process, inadvertent expansion first involves agents on the periphery of a state or empire acquiring territory without the authorization or knowledge of higher-ups. Leaders in the capital must then decide whether to accept or reject the already-acquired territory.Through cases ranging from those of the United States in Florida and Texas to Japan in Manchuria and Germany in East Africa, Anderson shows that inadvertent expansion is rooted in a principal-agent problem. When leaders in the capital fail to exert or have limited control over their agents on the periphery, unauthorized efforts to take territory are more likely to occur. Yet it is only when the geopolitical risks associated with keeping the acquired territory are perceived to be low that leaders are more likely to accept such expansion. Accentuating the influence of small, seemingly insignificant actors over the foreign policy behavior of powerful states, Inadvertent Expansion offers new insights into how the boundaries of states and empires came to be and captures timeless dynamics between state leaders and their peripheral agents.