Originally Posted by Natty
"Whenever any form of government is destructive of these ends [life, liberty, and the pursuit of happiness] it is the right of the people to alter or abolish it, and to institute new government in such form as to them shall seem most likely to affect their safety and happiness.
-- Declaration of Independence, 1776.
The means to alter our Perpetual Union were given: the amendment process. The means to abolish the Perpetual Union were also given: the Constitutional Convention. The South refused peaceful, legal means and appealed to force of arms. They made war on their nation, and that's the definition of treason.
After the rebellion failed, the guys on the ground chose mercy as the more healing approach, but that decision wasn't unanimous. They got forgiveness, not vindication.