The only substantial difference between them, and the one that divided them politically almost since birth, was their system of labor. - William C. Davis

Race had never been a defining element in successful nation states. The true definition always depended far more on distinctions in language, culture, and political institutions. Southerners spoke precisely the same language as Northerners, so there was no distinction there.

Southern states had embraced the Union only insofar as it served to protect their rights to hold property in slaves, and to spread slavery as the nation expanded and the institution itself became intertwined as a defining element in the struggle for national power itself. In slavery could not spread as new states were formed, then the existing slave states would be doomed to perpetual minority in representation in Congress, guaranteeing that if the day came when Northern antipathy to slavery itself became hot enough, the majority could use the government to subvert the Constitution and abolish the institution where it already existed. In short, the South could not affort to lose any battle over slavery, nor even over issues on its periphery.

Confederates were terrified of what was happening to slavery.