With good reason, the historian Bernard Bailyn – writing near the end of the twentieth century – described the “peopling of British North America” as “the most sweeping and striking development in this millennium of Western history.” The movement of people from one side of the ocean to the other “transformed at first half the globe, ultimately the whole of it, more fundamentally than any development except the Industrial Revolution.” He was not alone in believing this. Bismarck, as he reminded us, had called the migration of so many “the decisive fact in the modern world.” The movement of men and women from the Old World to the New, and this is fairly uncontestable, made British North America and then the United States distinctive, creating a diverse cultural landscape, just as the flow increased the productive capacity of the West in ways that no one could have anticipated. Migration produced untold wealth, expanded the territorial footprints of colonies and then states, remade the political economy and the social fabric of the broader Atlantic, and would make the United States one of the most powerful nations in the world. The story of migration is the story of America.