Berlin Wall

The Berlin Wall was a barrier which separated West Berlin from East Berlin and East Germany as a whole during the Cold War. Democratic uprisings in the Eastern Bloc countries resulted in the fall of the Berlin Wall on 9 November 1989 as well as the reunification of Germany and the collapse of the Soviet Union in the early 1990s. (AUDIO: Protect and Survive; COMIC: The Broken Man)

Prior to or during his tenth incarnation, the Doctor had witnessed the fall of the Berlin Wall on two occasions. The actor David Hasselhoff was also present and sang a song. (PROSE: Autonomy)

In 1903, after receiving a wealth of information from the future, Grigori Rasputin foresaw the construction of the Berlin Wall. (AUDIO: The Wanderer)

On 28 November 1989, Yuri Azarov, described Mikhail Gorbachev, the General Secretary of the Communist Party of the Soviet Union, as a "weakling [who sat] in Moscow and [watched] the Berlin Wall fall." (COMIC: The Broken Man)

Alternative timeline
In an alternative timeline created by the Elder Gods in the hope of destroying Earth, the militant hardliner Vladimir Kryuchkov became the General Secretary following the death of Konstantin Chernenko in 1985 rather than the considerably more moderate Gorbachev.

Significant international tensions began following the democratic uprisings in the Eastern Bloc countries. Kryuchkov instructed the protestors to disperse and, when they refused to do so, he sent in the tanks. The Red Army fired on the protesters, even the children, resulting in a massacre. Following the massacre, there were protests on the Berlin Wall, which resulted in the Soviet Union entering West Berlin on the pretext of restoring order. This meant that the American airbases fell into the possession of the Soviet Union. In response, the United States issued an ultimatum to the Soviets. The Western powers' satellites detected a Soviet army amassing on the border of West Germany, which was destroyed by an American tactical nuclear weapon on 6 November 1989.

Consequently, a nuclear war which came to be known as World War III broke out between the United States and its allies including the United Kingdom on the one hand and the Soviet Union on the other hand on 9 November 1989. Given that nuclear weapons were used by both sides, hundreds of millions of people were killed in the conflict. This timeline was ultimately negated by the Seventh Doctor. (AUDIO: Protect and Survive)

Behind the scenes
According to a sighting report sent to Whoisdoctorwho.co.uk by someone under the username of "rewboss", the Ninth Doctor was present for the destruction of the Berlin Wall.