Instead, secession (or "disunion") is always seen as a last resort, requiring mutual consent or material usurpations and oppression.
In other words, the Federal Government was not created as compact or contract between the states, but a legal framework ratified "by the people."
This claim will always seem absurd, since:
1. the constitution was ratified by the states 2. later states were not represented in it's formation
3. "the people" is an abstraction
4. Most important, it closes the door to secession in the future-- perhaps not that distantwhen it--or more practically its implicit threat--might be the last measure to forestall tyranny.
In other words, this reading will make it possible for the Federal Government to imprison or assassinate citizens it deems seditious, or mobilize the army against its own citizens-- (a violation of its own constitution.)
Those words, "We the people" do appear prominently in the Constitution, and we might note that ratifications were not made by state legislatures with governors approving, but rather by specially elected conventions of, yes, "We the people".
But, as for secession (or "disunion"), it was contemplated at the Founding much like a young couple might consider divorce on their wedding day -- yes, a theoretical possibility way off in the future, God forbid, but surely only through "mutual consent" or some major "oppression" or "usurpation" serious enough to rank as a material breach of contract.
No Founder condoned secession "at pleasure", meaning without serious necessity.
But in 1860 there was no "mutual consent" even sought after, and no "oppression" or "usurpations" even pretended by secessionists in their secession documents.
So they seceded, in our Founders' words, "at pleasure" which was neither Constitutional nor lawful.