You misunderstand the crime of treason, which is to say being a traitor. A traitor is, by very definition, a person who subverts the cause of his country while simultaneously professing to be its friend and operating within its structure.
It does NOT apply to persons who have openly shed their affiliation with a country and, where the two subsequently meet, have declared their opposition and hostility to it. Such a person may be an enemy and a belligerant, but he is not a traitor as he no longer professes friendship or affiliation within the country he opposes.
It is for this reason that Benedict Arnold and John Kerry were traitors - they both conducted their subversive acts while professing friendship to the target of those acts - but George Washington and Jefferson Davis were not.
Amazing how you guys just make things up as you go along.
Arnold would have been a traitor had he just deserted to the enemy and began fighting against the U.S.