So as a state you can voluntarily join to form a
government but then you can never leave it?
Sounds like Islam to me.
Except that for 37 of the 50 states there was no 'voluntarily join' to it. They were allowed in, and only after a majority of the existing states said it was OK through a vote in Congress. If the other states have to agree to let them in then why shouldn't they have to agree to let them out?
In historical fact, there was no Civil War, and President Lincoln announced in his inaugural address (March 4, 1861) that Confederates could not have Civil War unless they themselves started it.
Confederate President Jefferson Davis promptly issued orders for the seizing of Fort Sumter and raising 100,000 man Confederate army -- at a time when the entire US Army was around 15,000 mostly scattered in posts out west.