Actually the Yankees refused to leave Ft Sumpter, there is a right for states to leave the union. Therefore this was the War of Northern Aggression, they wanted to keep and dominate the South. Was slavery evil yes, but that doesn’t exclude the rights of succession for the states. I pray we have a peaceful divorce now rather than a bloody one later.
Fort Sumter belonged to the federal government of the United States of America. The South Carolinians had no claim to it.
There is no “right of secession”, although Madison did remark that there were two ways that a state cold withdraw: the same way they came into the nation (through congress) or through rebellion.