A just cause would be when the the government betrays its principles. Then the people of a state have a right to leave.
Was the south's cause a just one? I don't claim to know enough to answer that. People here seem to know a lot more about this than I do. I am just stating my general belief on succession.
I'm not sure what that means. Governments operate on laws. Nations can have 'principles' as embodied in culture or in the people themselves, but governments can only work to those principles within the law. In 1860, we had a "Nation Principle" that all men were created equal under the law. The "law" under which the government operated, however, said something else.