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.
Not hardly. Lincoln started a war with the South by an unprovoked act of aggression (larded with preparatory lies) against the independent State of South Carolina.
If the United States stood an invasion fleet into Tampico Bay, I rather imagine the Mexican Government and the local commandant would do the same thing Beauregard did.
The United States Government was no longer "established". It was constitutionally and lawfully disestablished by acts of the People of the South.
Its writ no longer ran in the South, the People having repudiated it, as it was and is their God-given right to do.