German army enters Paris in June, 1940

In June 1940, Paris was declared an open city as German forces entered, marking a significant triumph for Germany. This event echoed the 1871 siege by Bismarck's forces, but was accomplished by Hitler in less than a week.

What historical event is referenced by the mention of Bismarck's forces in 1871?

Compare the duration of the siege laid by Bismarck's forces in 1871 to Hitler's accomplishment in 1940.