The fact of history remains that neither President Buchanan nor President Lincoln lifted a finger to prevent Deep South states from first declaring secession, then forming a Confederacy, establishing a Confederate constitution, Confederate national government & military forces.
Both only acted (Buchanan ineffectively) to stop Confederate aggression against the United States.
Both Democrat Buchanan and Republican Lincoln believed that unapproved unilateral declarations of secession were wrong, legally, but neither believed the United States could take military actions on that basis alone.
So Civil War was not the result of declarations of secession, but rather of Confederate aggression against the United States.
Telling people to get out of a fortress that overlooks the entrance to one of their most important harbors is not an act of aggression against the United States. Burning cannons at one fort at which they had been told would be turned over to the confederates peaceably and sneaking into this other fort in the dead of night, are acts of aggression against the Confederates.