I was wondering about letters sent home from people in Japanese POW camps - how was the transfer of mail effected, given that diplomatic contact between the two countries would be strained to say the least.
Was it all handled by the Red Cross?
Since this seems to be lying unanswered …
Basically, yes. From this history of the Australian Red Cross:
As for how it worked, this page has a good selection of photos of documents relating to POWs in Europe. While none of the actual items of correspondence seem to bear direct evidence of the involvement of the Red Cross, this letter is an example of how the families were informed of the appropriate procedures for sending letters. In this instance, they were British and so they were told to send letters to the “Prisoners of War Department, St. James Palace, London”. This was a branch of the Red Cross. I don’t quite know what happened then. But the 1944 Nobel Peace Prize was awarded to the organisation and the corresponding Nobel Lecture goes into some detail about their activities during the war in Europe. While it doesn’t specifically mention letters, it does discuss how they distributed food parcels. Since families could send parcels to specific POWs, I’d guess that their postal network was rather similar.
On the other hand, there were other mechanisms. For example, in the case of Japan, radio broadcasts. As for Germany, there’s even this case.