Did international mail services continue to operate during World War I and World War II? Would it have been possible for citizens in two hostile countries to send personal mail to each other? If so, how was it accomplished – did hostile countries continue to exchange mail with each other directly, or would mail have had to go through a neutral country first? If it was not possible for hostile countries to exchange mail, then what made it impossible? That is, did governments typically ban their citizens from corresponding with citizens of hostile nations, or was it more a practical problem of the country’s postal service refusing or not being able to travel to hostile countries, for fear of the safety of its workers and equipment?
What about the corresponding with neutral countries which were surrounded by hostile countries? For example, was it possible in WWII to send mail from Britain to Switzerland? If so, what route did it take? Did the Royal Mail give such mail to the Deutsche Post (or the post service of one of the German-occupied countries) to forward to Switzerland? Or did it give it to some neutral country like Portugal, which would then forward it to the Germans, and then onto Switzerland? Or did the British have a special airmal service direct from Britain to Switzerland?
Diplomatic pouches, and the mail (I guess) went from neutral Switzerland through (combatant) France on a twice-weekly sealed train. Do not ask me how I know this. Trust me.
I work at a Primary School. One day, rummaging through boxes of old junk for a display on the history of our school, I came across a reel of Kodak film which was shot in 1942, showing the then layout of the school. That film had to be sent from Australia to the US to be developed, and then returned.
It took 18 months to get there and back, returning to the school in late 1944. So not even the peril of the Japanese Navy could stop the mail getting through back then!
I read recently that all international mail sent to the US was diverted through the Caribbean where it was opened and read. I’ve been looking unsuccessfully for a cite.
I suppose it might be possible for you to send a letter from, say Akron, Ohio, USA to Dresden, Germany, but very very difficult. Don’t know if it would be illegal, but in times of war, it very well might have been to correspond with foreign nationals in a country that is in open hostilities with your own country. I’m pretty sure the USPS wouldn’t be doing any business with the German postal service if for no other reason than that the two governments would have severed all of their relations when they declared war upon eachother, and I’m guessing this likely included their mail back and forth.