Between 1840 and 1914, roughly fifty-five million Europeans left for the Americas, Australia, and southern Africa. Roughly fifteen million Indians, Chinese, and Japanese left for plantation colonies, mining frontiers, and Pacific economies. It was the largest peacetime population movement in human history — at its peak, more than a million people a year crossed the Atlantic, packed into the steerage of steamships that had been carrying cargo a week earlier — and it remade the demography, the languages, and the cuisines of every continent it touched. Whole regions emptied: Italy, Ireland, and the Russian Pale sent away a fifth or more of their people, while New York, Buenos Aires, and São Paulo filled with their children, and the world quietly rearranged which tongues were spoken on which shore.
The European migrations were driven by several mechanisms working at once. Falling transport costs — the steamship cut the Atlantic crossing from weeks to ten days and the fare to a few weeks' wages, while railways funnelled inland villagers to the ports. Rising agricultural productivity meant fewer hands were needed in the European countryside just as a falling death rate kept birth cohorts large — a population boom with nowhere to work. Specific catastrophes pushed specific peoples: the Irish Potato Famine of the 1840s, which killed a million and drove a million more out, halving Ireland's population through death and flight; the Russian pogroms after 1881, which drove some two million Jews west. And cheap land pulled them — the American Homestead Act (1862) and parallel policies in Argentina, Canada, and Australia gave a settler 160 acres for the labour of farming it. Italian, Polish, German, Irish, Scandinavian, and Jewish populations remade New York, Buenos Aires, São Paulo, Toronto, Melbourne. The Asian migrations were largely indentured — Chinese labourers to Cuba and California, Indians to the Caribbean, Mauritius, and Fiji, Japanese to Brazil and Hawaii — bonded, often brutal, organized to replace the enslaved labour the abolition of slavery had withdrawn, and the seed of enduring diasporas. The flood ended abruptly with the First World War and the racially restrictive immigration laws of the 1920s, above all the U.S. quota acts of 1921 and 1924, which slammed a door that stayed shut for forty years.
The current age of migration — refugee flows, economic migration, climate displacement — is, relative to world population, smaller than 1880–1914, yet politically far more contentious, because the receiving societies are now demographically settled and treat their composition as fixed. Understanding today's migration politics requires knowing that the receiving societies were themselves built by migrations a century or two ago — that the nation defending its borders is often the product of an earlier open one. The historical amnesia is not accidental; forgetting is what lets the descendants of migrants pull the ladder up behind them.