Actually, they didn't, and actually, no, it isn't.
But then we've discussed the constitutional requirements of rebellion. You were paying attention, weren't you?
Well, yes they did and yes it was. They chose to initiate an armed rebellion by firing on Sumter. Rebellion is defined as "open, armed, and usually unsuccessful defiance of or resistance to an established government." The southern actions were open. They were armed. And they were certainly unsuccessful.
But then we've discussed the constitutional requirements of rebellion. You were paying attention, weren't you?
I sure was. You're just confused because I don't accept your interpretation of the Constitution. You seem to think that that is the same thing as being wrong.