Jefferson Davis sought to tbe tried for treason and was indicted. The US government delayed the trial on several ocassions and subsequently Davis was issued a full pardon.
There is nothing in the founding documents that says what he and the CSA Government did was treasonous. If he had gone to trial—he could have won. Many states considered Session before 1860—including New England (during the unpopular Mexican War). The Bloody Civil War should have been avoided if cooler heads had prevailed.