The Soviet Union began building the Berlin Wall on August 13, 1961.
Why did they build the wall?
The Berlin Wall was built to prevent citizens of East Germany (the German Democratic Republic or GDR) from escaping to West Germany (the Federal Republic of Germany or FRG). In the years after the end of World War II, more than 2.5 million East Germans had fled to the West. The Communist government of East Germany feared that the growing number of defectors would lead to the collapse of the East German state.