Per the rules of war, is the Commander-in-Chief a legitimate military target? Could we have assassinated Hitler?
JD doesn't qualify.
Sorry, but the Confederacy maintained diplomatic relations with the Vatican, and was addressed as the "Illustious and Honorable President" by the Pope. Regardless of your sentiment, President Davis was the head of the Confederate States of America.
I don't think a head of state is a legal military target. For a sufficiently evil head of state, like Hitler, I would break the law.
I'm sorry the Vatican extended diplomatic relations with Davis too, but if all you can cite is the Vatican, I think that means NO ONE ELSE extended diplomatic relations to Davis. So by your definition, I suppose the rest of the world is with me: JD doesn't qualify. If the Great Slaver Rebellion of 1861 had succeeded, diplomatic recognition most surely would have followed, but alas, it was not to be.
What rule of war is that?
That's a fallacy. No ambassadors were ever exchanged, and the Vatican Secretary of State has denied that any recognition was ever intended.