After WWI, cities become more racially and ethnically diverse due to immigrants and African Americans moving to cities to get jobs created by the war.
The Great Migration is a term that refers to the waves of African-Americans who left the rural areas of the southern United States during the nineteenth and especially the mid-twentieth centuries and moved to cities in the northeast, midwest, west and south.
The First World War ignited the Great Migration. The war effort led to increased demand for industrial products made in the North, but also a labor shortage. With the native-born and immigrant populations usually relied upon off at war, companies looked south.
Between 1900 and 1920, some 1.5 million blacks migrated from the south to northern cities (700,000 during World War I alone – between 1914 and 1918). Between 1920 and 1945, they moved from the south to northern and midwestern cities in even greater numbers.
See Priceonomics.com's The Great Migration: The African American Exodus from The South to know more.