Not strictly true. It never declared secession achieved through Congressional and Presidential measures as 'illegal'.
Though what was achieved in the civil war was a precedent, that were a state to secede without Congressional and Presidential measures in it's favor, then said state would not be recognized as an independent nation by the US government - as a result the Federal government would view such a state as either in open rebellion or in a form of public disorder (and could respond as empowered by the US constitution and Federal laws to 'bring it into line').
In other words, might makes right.
Do you turds ever wonder why people keep telling you that you're a bunch of Nazis?
Hardly. America is not an absolute democracy, meaning that if a majority in x state want to secede, they don't automatically achieve the right to do so.
Instead they have to go through the full process of appealing to Congress, and then the President.
Though if you are still butt-hurt since the civil war, here's a tissue:
There is nothing in the Constitution about a process for seceding. Your belief that such is required is purely a fantasy.
Sure there is. It is called the Congress and the President, who have the legal responsibility for legislation and treaties of the United States. There hasn't been an act of lawful secession as yet, so as a result it has never come up.
Secession isn't a treaty, dumbass. A number of times bills were submitted to Congress to make secession illegal and they were voted down. Apparently even the U.S. Congress agrees that secession is legal and constitutional.
Simply taking up arms and calling yourself independent, was never legitimatized by the US Federal government*, and the precedent was set after the civil war that no future acts of secession through similar means would be tolerated.
The fantasy is believing otherwise.
*Ironic given America's founding, but still a precedent set after the Confederacy sought to secede through military means.
Where does any document say it has to be "legitimized?" What you Lincoln cult turds have consistently failed to do is prove that there is the slightest evidence that the Constitution prohibits secession. The "precedent," as you call it, was a brutal invasion. Apparently the Nazi's set the "precedent" that it's OK to invade your neighbours and slaughter millions of unpopular minorities.
Apparently liberals posses the moral code of mass murderers.