In the book "Three Months in the Southern States" by Sir Arthur James Lyon Fremantle, a travelogue written by a Scottish writer as he travels around the Confederacy during the American Civil War, the writer interviews the Confederate Secretary of State, Mr Benjamin. Mr Benjamin makes the claim that:
[...] the Confederates were more amused than annoyed at the term "rebel," which was so constantly applied to them; but he only wished mildly to remark, that in order to be a "rebel," a person must rebel against some one who has a right to govern him; and he thought it would be very difficult to discover such a right as existing in the Northern over the Southern States.
Which seems like a reasonable point to me. So the question is: before and during the American Civil War, what LEGAL arguments were used to justify keeping the Southern states in the Union by force, and do those legal arguments stand up to scrutiny? Also, would the same arguments apply today to a state that wished to secede from the USA?