The dissertation explores how the federal government built, expanded, and employed imperial state structures and policies to acquire, govern, and incorporate territorial peripheries. Drawing on scholarship in history, sociology and political science as well as a wide range of primary sources, this study focuses on the state- and nation-building challenges that territorial peripheries posed to federal state officials and how they responded to them. State-building is defined as the enforcement of sovereignty claims and the establishment of governing authority. It is distinguished from nation-building defined as the state's management of inclusion and exclusion regarding civic and political membership in the core polity. The concept of tutelage captures the approach of the U.S. government to populations who were subjected to U.S. governing authority without sharing the same political rights, protections, and privileges of those residing within one of the states of the Union. Throughout the 19^th century federal government officials established governing authority over far flung territorial peripheries and regulated the status of diverse populations. Besides American settlers, territorial nation-building affected Native Americans, foreign born populations of Spanish, French, and Mexican descent, white southern secessionists, freedmen, and insular people overseas. The dissertation uses an historical-institutionalist approach, locating the origins of key U.S. institutions in the British Empire and tracing their development throughout the 19^th century. Premised on settler colonialism, the territorial system gave the federal government authority to create, govern, and incorporate western territories. The military defeat and conquest of the Confederacy turned the South into a territorial periphery that the federal government sought to reintegrate into the Union. The study ends with the examination of the acquisition of overseas territories at the turn to the 20^th century. The focus on the relationship between imperial state structures and American political development provides the basis for challenging prevalent notions of the exceptional weaknesses of the 19th century American state. Like other 19^th century states, the U.S. national government built state capacity to enforce its authority over expansive territory and the populations inhabiting it. Further, the dissertation shows how cultural and racial conceptions of the American polity shaped federal nation-building.