Industrialization encouraged migration wherever it appeared. The increasingly global economy globalized the labor market. The Atlantic slave trade diminished sharply after 1820, which gave rise to self-bound contract labor migration from Europe and Asia to plantations. Overpopulation, open agricultural frontiers, and rising industrial centers attracted voluntary migrants. Moreover, migration was significantly made easier by improved transportation techniques.
Transnational labor migration reached a peak of three million migrants per year in the early twentieth century. Italy, Norway, Ireland and the Quongdong region of China were regions with especially high emigration rates during these years. These large migration flows influenced the process of nation state formation in many ways. Immigration restrictions have been developed, as well as diaspora cultures and myths that reflect the importance of migration to the foundation of certain nations, like the American melting pot. The transnational labor migration fell to a lower level from 1930s to the 1960s and then rebounded.
The United States experienced considerable internal migration related to industrialization, including its African American population. From 1910–1970, approximately 7 million African Americans migrated from the rural Southern United States, where blacks faced both poor economic opportunities and considerable political and social prejudice, to the industrial cities of the Northeast, Midwest and West where relatively well paid jobs were available. This phenomenon came to be known in the United States as its own Great Migration.
The twentieth century experienced also an increase in migratory flows caused by war and politics. Muslims moved from the Balkan to Turkey, while Christians moved the other way, during the collapse of the Ottoman Empire. 400,000 Jews moved to Palestine in the early twentieth century. The Russian Civil War caused some 3 million Russians, Poles and Germans to migrate out of the Soviet Union. World War II and decolonization also caused migrations.
See World War II evacuation and expulsion and Population transfer in the Soviet Union for World War II forced migrations.
The Jewish communities across Europe, the Mediterranean and the Middle East were formed from voluntary and involuntary migrants. After the Holocaust (1938 to 1945), there was increased migration to the British Mandate of Palestine, which became the modern state of Israel as a result of the United Nations Partition Plan for Palestine.
Provisions of the Potsdam Agreement from 1945 signed by victorious Western Allies and the Soviet Union led to one of the largest European migrations, and the largest in the 20th century. It involved the migration and resettlement of close to or over 20 million people. The largest affected group were 16.5 million Germans expelled from Eastern Europe westwards. The second largest group were Poles, millions of whom were expelled westwards from eastern Kresy region and resettled in the so-called Recovered Territories (see Allies decide Polish border in the article on the Oder-Neisse line). Hundreds of thousands of Poles, Ukrainians (Operation Vistula), Lithuanians, Latvians, Estonians and some Belarussians, were expelled eastwards from Europe to the Soviet Union. Finally, many of the several hundred thousand Jews remaining in Eastern Europe after the Holocaust migrated outside Europe to Israel and the United States.