Why did the south secede from the union?

In duncan v. louisiana, the supreme court ruled that the [blank](fourth, fifth, sixth) amendment could be incorporated to the states. the [blank](equal protection, due process, enforcement) clause of the fourteenth amendment allowed incorporation to take place in this case.

What would lead a historian to consider two events to have a relationship of correlation rather than causation

1. what was the purpose of the communist international organization?