To my understanding protection under Geneva Conventions is only valid in officially declared wars.

The treatment of POWs under the Geneva Convention, as covered by the Third Convention of 1949, link here, states in Article 2:

In addition to the provisions which shall be implemented in peace time, the present Convention shall apply to all cases of declared war or of any other armed conflict which may arise between two or more of the High Contracting Parties, even if the state of war is not recognized by one of them.

Emphasis mine.

Honor Bound: American Prisoners of War in Southeast Asia, 1961-1973 actually talks about this: early in the war, North Vietnam wanted to try POWs as war criminals.

In fact, they paraded 52 POWs through the streets of Hanoi in the "Hanoi March" before tens of thousands of North Vietnamese citizens with civilians beating the POWs. Video here

A number of journalists were on site and covered it, and it led to condemnation internationally including from Indian PM Indira Gandhi and the UN Secretary General and Pope Paul VI. This included pressure towards the Soviet Union to restrain North Vietnam which led to North Vietnam backing off from their pledge to try them as war criminals.