Context is key to understanding a historic event, and contextualizing a complex event is a challenge. Where do you begin? Too far removed and the history will lack nuance. Too close and the history may present the event as particular when, in fact, it reflected a larger trend. Because the United States and Mexico were both new countries at the war’s outbreak in 1846, an overview of each nation’s colonial history is necessary. From there I studied each nation’s early history, looking at political, economic, and social developments. I examined the domestic concerns that animated people in both countries. Abolitionists in the U.S. fearing the expansion of slavery, or Mexican elites facing potential revolution and ethnic unrest. Or leaders in both nations fearing political adventurism in the continent’s undeveloped vastness. Concerns not unfounded. They were rooted in experience, in history.