Both the CSA and the USA had pro-slavery constitutions.
Both the CSA and USA had presidents who swore oaths to protect and defend their nation's pro-slavery constitutions.
In fact the USA president swore the oath twice.
In this context, it is important to remember the Confederate states were defeated by the most powerful slave-owning nation on earth.
And how many times did Jefferson Davis take the Oath of Office of loyalty to the USA?
My count is: six times, at least.
Of course, if you're a Southern Democrat, such oaths only mean what they mean when they mean it, right?