The United States is, and always has been, an empire. A host of recently published works expound the inseparable link between imperialism and the development of the United States and of its global standing. This dissertation aims to further this trend by examining U.S. imperialism in its key possession—the Isthmus of Panama. Few studies have explored the connections between the U.S. presence in the Canal Zone and growing U.S. control over the ostensibly sovereign Republic of Panama. Consequently, many consider Panama an atypical example of empire. Through multinational archival research, this dissertation offers a corrective. I examine how U.S. state and nonstate actors effectively made the Republic of Panama—and not just the Canal Zone—a U.S. colonial space, and, more specifically, a space typical of settler colonialism practiced around the globe. Through a process I call “spillover,” U.S. citizens settled and meddled throughout the isthmus, yet continued to take advantage of the Americanized institutions of the Canal Zone. Missionaries, soldiers, U.S. law enforcement personnel, agriculturalists, diplomats, and pleasure seekers expanded the imperial project set in Washington yet frequently relied upon U.S. government protection and interference. In this way, my actors on the isthmus closely reflect those agents of empire who conquered the “American West” or built colonies in the British and Japanese empires. My dissertation will, ultimately, show how U.S. citizens, through their “spillover colonialism,” set the tone for U.S.-Panamanian tensions during the mid-twentieth century and ushered in a new course for U.S. hegemony in the Greater Caribbean. I focus exclusively on the years between 1912 and 1936—after the construction of the canal but while Panama was still a U.S. protectorate. During this period, the United States learned new ways to impose its will abroad as empire, with all its costs and unpopular headlines, grew ever more cumbersome. Panama became the nursery for a brand of empire, rooted in my concept of spillover, where the United States could create colonies and control territory without going to war or planting the flag. My actors in Panama carried out all the hallmarks of imperialism, from occupying provinces to undermining Panamanian sovereignty with near impunity. At times, Panamanians pushed back, opting to resist U.S. incursions through diplomacy or more informal channels. U.S. imperial agents, therefore, learned how to apply their hegemony in ways that allowed them to sell their intentions as protecting U.S. lives and investments (such as the canal) and claim to observe the sovereignty of Panama. In so doing, U.S. colonialists in Panama effectively controlled a country of immense strategic value without having to wage the same campaigns that their countrymen had done in Haiti, Nicaragua, or the Philippines. These lessons bore fruit when, after President Franklin D. Roosevelt curtailed U.S. incursions in Panama in 1936, North Americans selectively spilled their cultural, economic, military, and political influence over the Atlantic-Caribbean territories that neighbored the bases, ports, and stations acquired from Great Britain in 1940. My project, carefully crafted into six chapters, brings Panama into the scholarly conversation on the typicality of U.S. imperial power abroad—something historians often use Puerto Rico or the Philippines to argue. / History