The Constitution does not mention words like "secession", "withdrawal" or "disunion".
Neither, for that matter, do state ratification statements.
Instead, states like New York and Virginia declared their powers could be "reassumed" if "necessary".
But the Constitution does clearly provide Federal powers in response to "rebellion", "insurrection", "domestic violence", "invasions", "treason" and "war".
The Constitution also provides numerous methods for states to resolve conflicts within constitutional limits.
Those include not just presidential elections, Congress and Supreme Court, but also the amendment process and even calling for a new Constitutional Convention.
Further, Founders were clear and consistent in saying that "disunion" could only lawfully come from mutual consent (meaning Congress) or from "usurpations" and "oppression" making secession "necessary" (i.e., confirmed by the Supreme Court).
In short: there had to be legitimate, actual reasons for secession, such as those they themselves endured from the British.
Founders never intended unilateral declarations of secession "at pleasure" to be constitutional.
So, non-constitutional secession would fall under the Founders categories of "rebellion" "treason" and/or "war".
But in November 1860, when South Carolina called its Secession Convention, there was neither "mutual consent" nor some Federal breach of constitutional contract to lawfully justify unilateral secession.
Indeed, there had been only one change from previous conditions: the recent constitutional election of "Black Republican" Abraham Lincoln.
And secessionist documents make clear: it was not some list of serious past grievances -- such as those in our Founders Declaration of Independence -- but rather their concerns over the future of slavery under those nasty, rascally Black-Republicans, which drove their Declarations of Secession.
In short: they declared secession unconstitutionally, "at pleasure".
But secession itself did not cause Civil War.
For five months after Lincoln's election, there was still no war, because the Federal Government still refused to respond to secessionists many acts of rebellion, insurrection and "domestic violence" against it.
Lincoln even announced in his first inaugural address that secessionists could not have Civil War unless they themselves started it.
Lincoln did not know that Confederate President Jefferson Davis had already issued orders to do just that, at Fort Sumter.
So when war came, it was not because of secession itself, but because secessionists themselves chose to start it.
The lessons for today are clear and simple: you will have a much better chance of protecting your vital interests within the law than outside it.
Rebellion, insurrection &/or war will result in the worst possible outcomes, from your perspective.
Only so long as there is a rule of law. When the law is selectively enforced, when specific groups get a free pass while others are prosecuted to the fullest extent, following the law is not the best course. While we have not completely reached that point, we are close. Even the Founders understood the necessity of throwing off the shackles of tyranny. If that cannot be effected within the law, it will be attempted outside those bounds.
That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness. Prudence, indeed, will dictate that Governments long established should not be changed for light and transient causes; and accordingly all experience hath shewn, that mankind are more disposed to suffer, while evils are sufferable, than to right themselves by abolishing the forms to which they are accustomed. But when a long train of abuses and usurpations, pursuing invariably the same Object evinces a design to reduce them under absolute Despotism, it is their right, it is their duty, to throw off such Government, and to provide new Guards for their future security.
We are nearing the point where this will again apply. Those who should be safeguarding our Liberty have become its worst enemies.
Yes, indeed. You're absolutely right.
But, look at it from the standpoint of the slaveholders. These are people who had become addicted to slavery. Often, their families had for generations been supported by the labor they stole from their slaves. They became trapped in a culture of dependency and that's why we hear the explanation that they "knew no other way of life." From the nursery, they were raised and cared for by slaves.
The slaveholders were parasites totally dependent upon their slaves for economic and personal support. Many of them talked about the need to end slavery, but most of them could not conceive of how they might make their own way in the world without the support of their slaves. So, they got lazy and did nothing and then, when they felt that their parasitic lifestyle was being threatened by talk of abolition, they very desperately tried to declare a "secession" so that they could continue to use the machinery of their State governments to protect their indolent lifestyles. The slaveholders felt trapped. They felt that they had no choice, no other option. So, that's what they did.
Most Southerners now are strongly opposed to slavery. Most now are very grateful that Lincoln and the United States freed the slaves and many now are also very grateful that Lincoln and the United States freed the slaveholders from their parasitic addiction to slavery. Granted, some of the slaveholders couldn't make it on their own and their lives could not be reconstructed. However, most regained their self-respect and moved on.
The Civil War was really just a case of very tough love.