In World War II, Japan and the European Axis powers had to
communicate over strategy and other things. Also, their
diplomats must have had to go to their own countries sometimes.
After the USSR entered the war, how was this done? Was this not
a major problem for the Axis Powers? Diplomats enjoy certain
immunities. When accredited as such, they can travel in "enemy
territory" without interference (unless declared persona non
grata), and only the least "civilized" countries will violate
the sanctity and privacy of diplomatic pouches. It was much
easier for Japanese diplomats to travel through the Soviet Union
than for German diplomats, because technically, the Soviet Union
and Japan were at peace, even though their respective allies
were at war with the other. Hence, most of the communication
between the Axis powers in World War II was done through
Japanese diplomats based in Europe.