Any state can legally secede from the union. Period.
So then, was the Civil War illegal?? That war forced the southern states to return to the union.
The Constitution is silent on it.
Use reason here. If states don’t have the option to leave, their people are not free.
Why would it be acceptable for an Estonia, Ukraine, Caledonia, Scotland, or Quebec to leave their unions, but not, say, a Delaware?
Of course it was illegal. Do you see a provision that says the national government can wage war on a state? However, winners write the rules and losers are forgotten.