The event that marked the beginning of the Great Migration of African Americans was the onset of World War I around 1914. The war created numerous job opportunities in Northern cities due to the demand for labor in factories, as many men were enlisted and sent to fight. This economic incentive prompted millions of African Americans to leave the agricultural South, where they faced segregation and limited opportunities, in search of better lives and greater social freedoms in the North. This movement significantly transformed urban demographics and cultural landscapes, laying the groundwork for future civil rights advancements.