In The Age of the Borderlands, acclaimed historian Andrew C. Isenberg offers a new history of manifest destiny that breaks from triumphalist narratives of US territorial expansion. Isenberg takes readers to the contested borders of Spanish Florida, Missouri, New Mexico, California, Texas, and Minnesota at critical moments in the early to mid-nineteenth century, demonstrating that the architects of American expansion faced significant challenges from the diverse groups of people inhabiting each region. In other words, while the manifest destiny paradigm begins with an assumption of US strength, the government and the agents it dispatched to settle and control the frontier had only a weak presence. Tracing the interconnected histories of Indians, slaves, antislavery reformers, missionaries, federal agents, and physicians, Isenberg shows that the United States was repeatedly forced to accommodate the presence of other colonial empires and powerful Indigenous societies. Anti-expansionists in the borderlands welcomed the precarity of the government's power: The land on which they dwelled was a grand laboratory where they could experiment with their alternative visions for American society. Examining the borderlands offers an understanding not just about frontier spaces but about the nature of the early American state— ambitiously expansionist but challenged by its native and imperial competitors.