Did Civil War settle the secession question?

Yes and no.

It is possible for a state to secced from the union if the proper steps are taken. But no state can just choose to leave the union on it's own. It requires permission.

1.  The state must request politely of it's own citizens to leave the union.

2. That state must take that request to Congress.

3. Two thirds of the other states must agree to let the requesting state secceed by constitutional amendment.

Failing that they could try to follow the Confederate's lead to exit by force. Their willing citizens would have to face the might of the US Military and face treason for inserection and taking up arms against the United States.

What state do you know of that could mount a military showdown against the world's greatest military? Go on Proud Boys...try it. We dare you.

The 1869 Supreme Court case Texas v. White 1 determined  secession was never a real thing in the eyes of the government. The Confederate States of America wasn't an independent country any more than your house is its own country simply because you say it is. "The Constitution, in all its provisions," the justices wrote, "looks to an indestructible Union composed of indestructible States."



1. https://www.law.cornell.edu/supremecourt/text/74/700