National immigration policies vary considerably. In Europe the process of integration has meant that European Union member countries permit the free movement of citizens from other EU nations. But in many other cases, restrictions on migration have increased even as barriers to trade and investment have fallen.

The border between the United States and Mexico reflects many of the challenges of contemporary migrations. Since long before a border existed between the United States and Mexico, migrants have circulated throughout North America, but as the United States conquered land that had belonged to Mexico in the nineteenth century (see Chapter 27), it restricted the movement of migrants northward across the border. At the beginning of the twenty-first century the U.S. government began building a wall at its border with Mexico, further restricting the circulation of people even as the United States and Mexico implemented a free-trade agreement that made it easier for goods and capital to cross the border.

Similar trade agreements, coupled with similar restrictions on migration, exist between the United States and many Central American and Caribbean nations. The United States is by far the leading source of foreign investment in those countries. And the United States has participated in military conflicts in all of those countries over the past century, often multiple times. Still, within the United States, the most intense public discussions about Latin America surround the restriction of migration.

In many cases, restrictions on immigration have increased in countries where national economic growth has slowed. For instance, as Japanese industry boomed in the 1980s, the country welcomed descendants of Japanese emigrants who had settled in South America in the first half of the century. Because these migrants were culturally and linguistically different from natives, Japanese citizens considered them dekasegi, or “temporary guest workers,” who had no right to citizenship despite their ancestry. As manufacturing and economic growth stagnated in the 1990s, this circuit of migration to Japan dwindled. During the same period millions of people, first from South Korea and then Vietnam, Cambodia, and Laos, whose countries had experienced great upheavals in conflicts involving the United States found legal refuge in the United States. (See “Individuals in Society: Sieng, a Mnong Refugee in an American High School.”)

Though pursuit of economic opportunity and flight from persecution are the major factors that drive international migration, other factors shape the creation of migratory circuits. A migratory circuit is a deep connection created between two regions through an initial experience of migration that results in a greater circulation of people. These migratory circuits have often followed experiences of violence. For instance, many immigrants settling in European countries like Britain or France migrated from regions that those countries dominated as colonies. In turn, circuits of migration to the United States have often followed U.S. military actions, such as wars in Southeast Asia and Korea or military interventions in Latin America. It remains to be seen whether U.S. military action in Iraq and Afghanistan will eventually yield the same result.

Migrants usually become ethnic, religious, or linguistic minorities in the countries where they settle, and they commonly face discrimination. Sometimes this discrimination is expressed in violence and oppression, such as that experienced by contemporary Zimbabwean workers in South Africa. In turn, discrimination and social pressures have also triggered violent reactions from ethnic minorities, such as riots in the Paris suburbs in 2007 and Stockholm in 2013 to protest discrimination experienced by Middle Eastern and North African migrants, or the radicalization of some Muslim migrants in Britain. When migrants lack legal standing, as is the case for millions in the United States, they can fall prey to criminal organizations or be exploited by employers.

Migrants don’t just cross national borders; they also move within countries, most often from the countryside to the city. In countries with high degrees of rural poverty combined with a significant amount of urban industrialization, this current of migration often leads to sprawling, overcrowded urban areas.