Complex sentence: Mississippi was part of the Black Belt, and had a majority-black population from the antebellum years until the 1930s, after which in the Great Migration, nearly 400,000 African Americans left the state for opportunities in the North, Midwest and West.
Split into simpler sentences: Mississippi was part of the Black Belt, and had a majority-black population from the antebellum years until the 1930s. During the first half of the 20th century, a total of nearly 400,000 African Americans left the state during the Great Migration, for opportunities in the North, Midwest and West.