“Our position is thoroughly identified with the institution of slavery—the greatest material interest in the world.”
Seems reasonably clear.
Yes the CSA had a pro-slavery constitution. So did the USA.
Yes the CSA president took an oath to uphold his nation's pro-slavery constitution. So did the president of the USA; twice.
Yes the CSA was created after unilateral secession. So was the USA.
The CSA was probably the second most powerful slave-owning nation in the world but it was defeated by the very most powerful slave-owning nation in the world.
Do you know which nation that was?