I can't see the original Spanish veterans agreeing to serve as torturers, or even many SDS recruits (the Weathermen usually tried to avoid killing anyone). If people like the ones who formed the Red Brigades or RAF in Europe joined up, though, it might be a different story - but they couldn't be tried for treason.
Actually, what would be the legal status of, say, an Italian who went to Vietnam as a volunteer and was captured? Today, jihadis who do this kind of thing (Syrians in Iraq, etc) are considered illegal combatants by the US military - criminals. Did this always apply?