There were many mass migrations in the Western world throughout the nineteenth century, and they had many different causes. The main cause, however, was economic. People in various places throughout Europe felt they had better economic prospects around the world, particularly in the Americas, than at home.
In some cases, like the famine of the 1840s in Ireland, there was hardly a choice. Over one million Irish people left their homeland—most destined for North America, though many went to English and Scottish urban areas as well. Similarly, immigrants from southern Italy fleeing rural poverty made their way to the United States and South American countries like Brazil and Argentina in the late nineteenth and early twentieth centuries.
Many eastern European immigrants came to American cities for basically identical reasons. Most were peasants or lived small villages, and they came seeking economic opportunities. Some immigrants came fleeing social unrest at home. Many Germans came to the United States and elsewhere in the Americas in the wake of the turmoil of the mid-nineteenth century revolutions that rocked German principalities. Ethnic minorities, particularly Jews, fled persecution in Russia and Austria-Hungary in the late nineteenth and early twentieth centuries. They landed in large numbers in the United States and also in Great Britain and other places in Europe.
All in all, these migrations were also made possible by economic growth in the United States, which created considerable demand for labor, widespread European investment in the Americas, and improved transportation and communication that made migration more logistically feasible. The consequences of migration included economic expansion in the United States, the spread of European influence around the world, and rapid urbanization in cities in Europe and the Americas.