There were no secessionists in 1776, only rebels, hence that annoying title, "The Revolution of 1776."
The Confederacy, even before the firing on Ft. Sumter, had already committed numerous war-like acts, while the US did nothing for months in the face of such aggressions. The reverse is true in that the United States finally had to defend itself from an illegal rebellion.
There was no "territorial integrity of the Confederate States" a claim never recognized by any other nation on the planet at the time. The Confederacy was considered an area in rebellion much to the frustration of Davis and other rebel leaders. It got so desperate for foreign recognition that it finally expelled all British counsels because England would not offer sovereign recognition to them.
What you really had was a bunch of arrogant slaveholders leading a region of the United States in order to protect, defend, even expand, slavery in the face of a newly elected Republican administration, whom they feared would interfere with that institution.
The Constitution and a free and fair election meant nothing to them, so we can dispense with the fantasy they were doing such treason by way of the Constitution.
This was a rebellion, illegal and ill considered, and doomed to failure and it's results are exactly where they should be.
On the ash heap of history.
Unionblue