As a Parisian, I've learned a lot about the second time the German army occupied the French capital during the Nazi invasion of WWII. But what about the first time the Germans took hold of the city of Paris? One forgets they managed it twice. The winter of 1870-1871 was a traumatic time for Parisian
The details are more-than-mildly interesting and have several valuable lessons for the modern day: https://en.wikipedia.org/wiki/Causes_of_the_Franco-Prussian_War