Ballooning Over Bullets, and Other Escapes From the Siege of Paris - the low countries
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When Paris was besieged by Prussian troops in 1870, the French used hot air balloons to transport mail and people across enemy lines into Belgium and the Netherlands.

I was aware that news and letters were brought into the city by carrier pigeons with microfilm, but I wasn’t aware that news and post from the city(along with the carrier pigeons themselves that would later return) were brought out by constructing one way balloons out of the diminishing resources for a harrowing flight over occupied France with a shrinking pool of pilots.