13 août 1961 - Building of the Berlin Wall

Description:

As one of the most powerful and long lasting symbols of the cold war, German Democratic Republic (or East Germany) started to build the wall called “Antifascistischer Schutzwall,” or “antifascist bulwark,” to keep Western fascists from entering East Germany.
This splitting of people and scientists and restricting of communication slowed the progress of science in Germany.

https://www.history.com/topics/cold-war/berlin-wall