During the German occupation in Poland in 1939-1945, Nazis authorities banned the breeding of pigeons. They feared that pigeons would be able to bypass German armies and deliver messages from other countries to Poland or messages from resistance. But the most, Nazis were feared that the world would get know what happens in Poland.

Author: Andreas Trepte under:Creative Commons Uznanie autorstwa – Na tych samych warunkach 2.5

Normal post office wasn't closed, but letters were censored. For example it was not allowed to use some forbidden languages like Hebrew. Letters were also not delivered to the countries which were not German's allies.The importance of carrier-pigeon was then great. They were used mainly by the Polish resistance movement and they played significant role in transmitting secret informations

Author: Harper's New Monthly Magazine, No. 275, April, 1873.