Yes, it would have been a trial on the legality of succession, and not a question the north wanted answered. If he was found innocent that would have meant they just spent the last 4 years killing people and ravaging the South without any legal justification.

Interestingly, after he was held without bail for 2 years, a bunch of prominent Northern businessmen pooled their money to bail him out; people who didn't share any of his beliefs but felt that he was being denied due process and that was wrong. I have often said that in the past we were a more principled people, and that sort of thing demonstrates it.