When nations went to war, they broke off diplomatic relations and interned enemy diplomats. In 1980, when the Iranian government seized American diplomats and held them hostage, this shocked people worldwide because the most bitter enemies had observed the principle of inviolability of diplomats for centuries. Even the Nazis did not violate the persons of diplomats including Soviet diplomats. Nor did the Soviets violate the persons of German diplomats after the two countries went to war. That the Iranian government violated the one international law almost universally respected, showed what a dangerous and outlaw régime they were and continue to be.
Since every country in World War Two had nationals in countries they were fighting, most belligerents named a “protecting power” to look after its citizens in an enemy country. To my knowledge, every Western country named Switzerland as the “protecting power”. Therefore, POWs in the West made complaints about their treatment to Swiss diplomats. (Since most officials of the International Red Cross were Swiss, the two groups blurred together but they were separate.)
German POWs in the US complained to Swiss authorities that while being taken to their POW camps, they were awakened by porters offering them coffee on the train and this disturbed their rest. That’s too damn bad. Others complained at being forced to sit in chairs in the passenger coaches. That was uncomfortable. Given that most German troops in Germany were transported in freight cars, this is a little much. Once at the POW camps, a standard complaint was American white bread which the Germans said was like cake and bad for them. So was starting World War Two you jackasses.