Why do states sometimes decline profitable opportunities to annex their neighbors? My dissertation answers this question by examining the domestic political effects of international territorial annexation. In short, I argue that state leaders consider domestic political consequences in addition to material benefits and military costs when determining the desirability of annexationism, and reject opportunities to annex territory where they fear it impairing their domestic influence (even where annexation would be materially profitable). I argue that such fears lead democracies, which are particularly vulnerable to swings in the domestic balance of power resulting from annexation, to reject the absorption of neighboring societies. I suggest that the current stability of the international system is a result of this mechanism, rooted in the spread of democracy among the great powers.
Testing this theory has involved two types of research. First, I constructed an original dataset of 288 annexationist ventures conducted by the modern great powers throughout their histories as powerful states. Using this dataset, I conclusively established both the decline of annexationism after World War II and the lack of annexationism by democracies. After laying out my theory of annexation?s domestic political consequences, I test its causal logic against the conventional wisdom by process-tracing the decision making of US leaders facing major opportunities to expand into Canada and Mexico between 1774 and 1871. The unique geopolitical features of the United States should have made it particularly desirous of conquering its neighbors, yet these case studies clearly demonstrate that US leaders desired the lands of their neighbors but not their populations because they feared the domestic political consequences of assimilating them.