Yes, I found this related answer to a question on r/askhistorians about letters from prisoners of wars: https://www.reddit.com/r/AskHistorians/comments/55p22q/h

Yes, I found this related answer to a question on r/askhistorians about letters from prisoners of wars: https://www.reddit.com/r/AskHistorians/comments/55p22q/how_were_letters_from_pows_delivered_during_ww1/d8chimv/ https://www.reddit.com/r/AskHistorians/comments/55p22q/how_w... "letters from and to POWs were usually sent through non-belligerent countries and humanitarian organizations such as Switzerland or the International Red Cross"