Forced international migration has historically occurred for two main cultural reasons: slavery and political instability. Millions of people were shipped to other countries as slaves or as prisoners, especially from Africa to the Western Hemisphere. Wars have also forced large-scale migration of ethnic groups in the 20th and 21st centuries in Europe and Africa. Another push factor would be the fear of prosecution and these people would be refugees: people who have been forced to migrate from their homes and cannot return for fear of persecution.