The Declaration of Independence makes it clear that the only reason a People can disband their government is when the government becomes destructive to the life, liberty and pursuit of happiness of the individual.
That was not the case for the South, who had no just reason to leave the Union.
And who is the determiner of whether or not their government has become destructive to their life, their liberty, and their happiness?
Is this up to the Government to decide, or is it up to the people to decide?
If you say it is up to the people to decide, then that is exactly what they did. They decided the existing government was destructive to their life, their liberty, and their happiness, and so they chose to leave it, just as the colonists chose to leave their Union four score and seven years earlier.
That was not the case for the South, who had no just reason to leave the Union.
The Union had no just reason to compel them to remain. People have a right to independence for whatever reason they wish it. To force someone to serve you when they don't want to, is slavery. Aren't you supposed to be against that?
Aren't you supposed to be against coercion? Against forcing people to work for you against their will?
So if you are against compelled servitude, then by what right do you claim to enforce servitude on the Southern States who merely wanted to be free of the others?