Why was the Berlin Wall built?

After World War II, Berlin was divided, and West Berlin became an island of the West inside communist East Germany. Around 3.5 million East Germans had left before 1961, draining the country of skilled workers. On 13 August 1961, the East German government sealed the border overnight with barbed wire, later replaced by concrete walls. It officially called the wall the 'Anti-Fascist Protection Rampart'.