Why was Berlin divided by a wall?

To stop people leaving. Between 1949 and 1961 something like 3.5 million people - a fifth of the East German population, and disproportionately the young, the educated and the skilled - simply walked into West Berlin, which sat as an island of the West inside East Germany and had no sealed border within the city. The state was bleeding the people it needed to function.

It went up overnight on 13 August 1961, initially as barbed wire strung by soldiers while the city slept, and hardened over the years into a system rather than a wall: two walls with a lit strip between them, watchtowers, dog runs, tripwires and orders to shoot. At least 140 people were killed trying to cross it. The official East German name for it was the Anti-Fascist Protection Rampart, which is one of the more transparent pieces of naming in the twentieth century.

It came down by accident as much as anything. On 9 November 1989 an East German official, handed new travel regulations at a press conference and unbriefed on when they took effect, said they applied immediately. It was broadcast live, crowds went to the checkpoints, and the guards - with no orders and no wish to fire - opened them. The wall had stood for twenty-eight years and was breached because of a badly answered question.