A major element which led to the Harlem Renaissance was the Great Migration during and shortly after World War I when large numbers of Black Americans moved to the North.Between 1910-1920, the Southeast lost 323,000 blacks; five percent of the native black population. By the end of 1920, 13% of the black population had moved north. Between 1910 and 1930, over one million blacks moved North. With this blacks slowly but surely gained political leverage by concentrating in large cities in states with many electoral votes. In the North they could speak and act more freely than in the North. Along with political activity came a spirit of protest that was expressed in a literary and artistic movement which became known as the Harlem Renaissance. The first significant writer of the time was Claude McKay, a Jamaican immigrant, who wrote a collection of poems known as Harlem Shadows. Among the poems were works such as "If We Must Die," and "To the White Fiends." Other writers included: Langston Hughes, Zoral Neale Hurston, and James Weldon Johnson who portrayed the black mecca in Black Manhattan. Johnson is perhaps best known as the author of "Lift Every Voice."
A substantial element was a movement known as Negro Nationalism, largely the work of Marcus Garvey, which exalted black cultural expression. All of this, or course, was part of the great upheaval of the Roaring Twenties when all previous standards were questions and the New Woman appeared as well as the "New Negro" to use the phrase of the time.
The Harlem Renaissance was caused by a combination of the black migration to the North (the "Great Migration") during and after WWI and the economic and social boom times of the "Roaring '20s." These factors made it possible for an upwelling of literary and cultural expression among African Americans that was centered in Harlem.
The Great Migration brought large numbers of African Americans to Northern cities. New York, of course, was the major center for those who were artists and writers and other sorts of intellectuals. By coming to New York, they were freed from many of the sorts of pressures and intimidation that kept them subjugated in the South.
The Roaring '20s provided the atmosphere of social ferment that helped to inspire the Harlem Renaissance. It was a time when social values seemed to be changing very fast and people were living life in new and more exciting ways. This atmosphere of cultural change also inspired artists and writers to examine their society in new ways.
The presence (for the first time) of large numbers of African Americans in the North, combined with the economic and social changes of the 1920, allowed the Harlem Renaissance to occur.