Try again. Davis could have been tried for treason, perhaps should have been tried for treason, but the same Chief Justice of the Supreme Court who would later rule that secession as practiced by the Southern states was unconstitutional also said that trying Davis for his crimes after the ratification of the 14th Amendment would have violated his 5th Amendment protections.
So much for your "they couldn't try him without proving secession was legal" nonsense.
Too bad we didn't aquire Guantanamo Bay until after the the illegal Spanish American war. Another one that we started just to be the bastards of the world that we've always been, and then covered up in the history books, by the way.
Can you point out exactly where the federal government’s power to use force to stop secession is enumerated in the constitution? I have not been able to find it myself. Thanks.