[Non-Sequitur]

4ConservativeJustices was obviously referring to the quote by Stanton to Chase, saying effectively that the Constitution does not equate secession with treason and, therefore, the Federal government couldn't convict Jefferson Davis.

Stanton was the Secretary of War, not the Attorney General or the Chief Justice. His opinion was one of may. Had Davis been tried he would have been convicted, no other outcome would have been conceivable to the Johnson administration.

[HenryLeeII]

Had Davis been tried he would have been convicted, no other outcome would have been conceivable to the Johnson administration.

...or no other outcome would have been allowed...