There were at least three major factors or events that led to the Harlem Renaissance.
First, there was World War I. This war led to the Harlem Renaissance in two ways. On the one hand, it gave many African Americans some experience of life outside of the United States. This opened their eyes to new perspectives. On the other hand, the war brought about a movement of many African Americans to cities in the North. Once there, they were freed from many of the constraints of life in the more racially oppressive South.
Second, there was an influx of African Americans into Harlem. Part of this influx was from the “Great Migration from the South. However, some of the people moving into Harlem were from the Caribbean. This, too, added to the broadening of horizons. It also helped to create a sense of pan-African unity and identity such as was promoted by Marcus Garvey.
Finally, there was the “zeitgeist” of the 1920s. This was a time when American society as a whole was in a boom period. It was a time of excitement and new ways. This would have helped promote the idea that African Americans, too, could experiment with new cultural forms.
All of these factors helped to bring about the Harlem Renaissance.