“Absent either mutual consent or necessity, our Founders considered secession to be “at pleasure”, which was not constitutional or lawful, but very close to rebellion, for which they gave no approval.”
But that was not the case in 1861. Read what a leading historian has found: “. . . nearly all 1861 Unionists (including Lincoln) believed that had the Deep South Fire Eaters contented themselves with declaring secession and setting up a Confederacy, the Union could not use force to stop them.”
And that is exactly right.
In 1861 Unionists believed at the same time that 1) Fire Eaters had no constitutional authority to declare secession "at pleasure", but 2) also that the Federal government could not use force to stop them, unless they themselves started war.
In today's wording, we would say they operated under a "no first use of force" rule of engagement.
But once the Confederacy began using force (i.e., at Fort Sumter), then Unionists felt free to respond appropriately.