[fortheDeclaration]

as per the Rhodesian Unilateral Declaration of Independence of 1965 from the British Empire, or that of India and Israel from their previous colonial master.

The Southern states were members of the Union, they were not part of a colonial system.

[M. Espinola]

Excellent point, and bloody obvious to the normal mind.

[lentulusgracchus]

The Southern states were members of the Union, they were not part of a colonial system.

Operative word: were. The Southern States seceded from the Union. They left.

And you are right: they were not part of a colonial system: they were sovereign entities, entitled to make their own future as they saw fit.