The government is given the power under the Constitution to suppress insurrections which is exactly what Lincoln was dealing with. Armed bands of terrorists attacking US installations is an "insurrection" and Lincoln put that one down. No one need pay any attention to those attempting to defend and justify such insurrections even when glorified by the cockamamie concocted concept of secession.
There was no mention of this in the body of the Constitution because the concept was so unthinkable that the Founders NEVER once discussed it during the Convention. They ALL believed the Union to be perpetual.
Well then they made a mistake. If they wanted to assure it was "perpetual" they should have included a prohibition of secession in the document.
And they DID discuss it.