The Great Migration refers to the relocation of hundreds of thousands of African Americans from the rural areas of the South to urban areas in the North during the years between 1915 and 1930. Although many of those who left the rural South migrated to southern urban areas, most migrants moved to cities in the North. It was the largest movement northward and into cities that had occurred among African Americans to that point in history. The United States’ entrance into World War I in 1917 played an important role in this movement, as the demand for additional labor grew in war-related industries at the same time that white workers were siphoned off to serve in the armed forces. Immigration also slowed dramatically, removing another source of labor for American industry. African American labor was one of the key alternative sources sought by these industries to enable them to respond to the growing demand for war-related goods. Industrial jobs that had not been previously available to African Americans now became accessible in greater quantity and variety. This flood of African American migrants dramatically changed the demography of many cities in both the North and South, as the percentage of African American residents exploded. Cities like New York; Philadelphia and Pittsburgh, Pennsylvania; and Chicago, Illinois, saw their African American populations grow by 50 percent or more during this period. This population surge placed great pressure on the municipal services and housing supply of these cities. It created growing tension between residents as they competed for places to live and for jobs, particularly after the war ended. As a consequence, the Great Migration pushed issues of race more to the forefront in the North. It also heightened these issues for the South as concern increased about the loss of workers in rural areas and the presence of growing African American populations in some of its cities. The movement added greater impact to a statement made by the African American intellectual W. E. B. Du Bois, who posited in 1903 that one of the critical issues of the twentieth century would be the question of the color line.