The shading indicates what percent of the county's population was enslaved, with darker shades indicating a higher percentage of enslaved people. As you can see, some counties along the coasts and near the Mississippi have slave populations as high as 80 percent. The map also included information on the overall population and percentage enslaved on a state level. You can see that image here. The numbers are astounding, to say the least; in Mississippi and South Carolina, slaves were 55.1 percent and 57.2 percent of the population, respectively. In raw numbers, Virginia had the largest population of slaves -- 490,887 enslaved persons -- but they were "only" 30.7 percent of the total population.
You can safely say that fear -- as much as ideology -- drove Southern planters away from the Union. If anything else, emancipation meant a huge population of former slaves, and to many planters -- who knew of past slave rebellions, at home and abroad -- this was nothing less than an existential threat. The same is true of the South's rapid descent into Jim Crow and racial terrorism following the end of Reconstruction. With so many former slaves, control was the first item on the agenda.
-- Jamelle Bouie