I do not buy the argument that unilateral secession is clearly forbidden in the US and I don't think the American Civil War settled that point. The Confederates stood a decent legal chance to secede from the Union. It was a constitution crisis. Instead the Confederates choose to throw constitutional avenues away and engage in a war of aggression against the North. The South didn't need to raise armies and launch an attack on Fort Sumter. However personally I'm grateful for their gross incompetence both in starting the war and their conduct throughout the war because it brought about the end of Chattel Slavery in North America.

Furthermore I would argue there is a large legal and ethical difference between:

1/ a largely peaceful movement for secession which is suppressed and then responds with riots

2. and a landed gentry rising up in open war and invasion because they lost an presidential election and they want to preserve their right to strip freedom from their countrymen.