So if there was one American serving in the Wehrmacht, or the Kriegsmarine, or the Luftwaffe during WWII, the U.S. government's military would have had no authority to shoot back at him without a trial first?
False context. The discussion is around domestic assassination (specifically of an unarmed journalist), not world war or war on a battlefield. Radically different concepts historically both from a legal and constitutional stand point. Your example couldn't be further away from the issue of assassinating an unarmed, non-threatening US citizen (whether domestic or overseas).
