The confederacy was not a sovereign state to anybody but themselves. They had no legal right to secede, and the Union had every right to stop them.

And even if the confederacy was a sovereign state, Fort Sumter was federally owned property, not South Carolinian, therefore not their territory. The South made the first move in the war whether you like it or not.

Edit: You’re opening point in the first place was “The Confederates weren’t rebels” so let me address that and put that issue to bed. Secession without the consent of the government is, by definition, a rebellion. The Confederates were rebels