Read what the states who left the union had to say about it: https://www.battlefields.org/learn/primary-sources/declaration-causes-seceding-states https://www.

Read what the states who left the union had to say about it: https://www.battlefields.org/learn/primary-sources/declaration-causes-seceding-states https://www.battlefields.org/learn/primary-sources/declarati... It was slavery. An excerpt from one state: "Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world. "