The era from 1830 to 1910 witnessed massive transformations in how people lived, worked, thought about themselves, and struggled to live and thrive. It also witnessed the birth of economic and political institutions that still shape our world. America's population grew more than ten-fold. The country expanded to the Pacific coast and then out into the Pacific itself. Civil warfare erupted. Three centuries of slavery ended. Native peoples were suppressed and remanded to reservations. Massive waves of immigrants from Europe and Asia arrived. From an agricultural society with a weak central government, the US became an urban and industrial society in which government assumed a greater and greater role in the framing of social and economic life.

A Nation Without Borders's signature achievement is to place this history in a wholly new light, by capturing the tensions and contradictions between nation and empire. His new interpretations include the fact that slavery was national not sectional, that Jim Crow racism initially emerged in the Northeast and Midwest, that "sectionalism" was less a fact of politics than an important political construct in the battle over slavery's future, and that the principle struggle of the period was not between the North and South but rather between the Northeast and the Mississippi Valley for control of the continent and hemisphere. Hahn's strikingly original thesis resets the familiar framework of that of a country expanding from colony. He shows how, the United States had, from its colonial origins and birth as a union, significant imperial ambitions on the continent and in the hemisphere, and that the United States only became a nation, a nation-state - as many others did - in the midst of a massive political and military struggle in the 1860s. A nation is understood to have clearly defined borders, which delineate sovereignty and belonging. Empires, though they may expand and contract, lack real borders; they are more about vectors, claims, and alliances. By 1910, as the "long" nineteenth century came to a close, the United States stood as one of the most formidable empires on the globe.