from JTF
By this discussion, the Gulf of Tonkin Resolution and the authorization to use force against Iraq (2002) constitute declarations of war.
Briefly, the 1949 GCs moved away from defining hostilities in terms of "war" to defining them in terms of "armed conflict". The GTR was more phrased in terms of Southeast Asia as a whole, where Vietnam was only one of the dominoes. If we take it in that broader context, we won the "armed conflict" as defined in that "declaration of war".

from JTF
I also believe that I understand your argument to say that there is no reason that a protected combatant who commits a crime - not inherent in making war such as premeditated murder of noncombatant civilians - could not be prosecuted for that act. (That is the Nurnberg precedent, isn't it?)
Combatant immunity applies only to a protected (lawful) combatant in reference to acts which are within the bounds of the laws of war. I did not make that clear enough in my screed. E.g., shooting a prisoner or a known non-combatant is an obvious crime in hindsight - what it seemed at the time to folks in the field was another thing entirely.