What historical event to spark the harlem renaissance and how

World War 1.

Many blacks left southern states during WW1 and moved to big northern cities to work in war industries.
More went to New York city than to any other,and after the war there a lot of blacks living in Harlem who were literate and artistic.

The  pan-african movement  was an "emotional, cultural, psychological and ideological  movement  that began among the  african  dispora in the western hemisphere, for a closer  purpose, so that  african  people could feel secure, attain political, economic as well as psychological power visa vis other races or world regions" .