The fall of the Berlin Wall - that unique moment in history when the most powerful symbol of the Cold War was peacefully made redundant, literally overnight. The fall of the Wall was only one of a long chain of events, both peaceful and terribly violent, that marked the moment that time had run out for the Soviet-supported dictatorships in Eastern Europe. 
From what seemed to be a state of permanent Cold War, one by one the Eastern European countries discarded their dictators. The GDR was especially loyal to the Soviet Union and the 1961 Wall surrounding Western Berlin, the 'Iron Curtain' along the West German border and the many GDR citizens killed trying to flee are proof of how serious Erich Honecker and other hardline leaders were about staying in power.
Mikhail Gorbachev changed all that when he called for openness and took away the threat of Soviet intervention. Poland was the first country to wobble, when the Solidarity movement signed a power-sharing agreement with the government in early 1989, and winning the elections in June. Hungary was next in line, when its liberal leaders dismantled the 200 kilometre stretch of barbed wire along the border with Austria, ignoring loud protests from its neighbouring Socialist countries. In May, the GDR government blatantly manipulated the outcome of the local elections, creating a sense of unrest, and that summer tens of thousands of GDR citizens holidaying in Hungary took the chance to escape via Austria to West Germany. Thousands more taking refuge in the West German embassy in Prague were allowed to emigrate on a train passing through Dresden in the GDR, and unrest along the route caused further embarrassment to Honecker. During the GDR's 40th anniversary parade on October 7, the crowds appealed publicly to Gorbachev to intervene. Two days later in Leipzig, the traditional Monday peace prayers in the Protestant church turned into a peaceful demonstration, and 70,000 brave citizens marched, surrounded by hundreds of armed riot police and incognito secret police who thankfully chose not to intervene. The following week 120,000 people showed up, and 320,000 the week after. The Monday demonstrations caused the SED party to replace Honecker with Egon Krenz on October 18, but these reforms came too late to stop the movement, and on 9 November SED official Günther Schabowski announced the relaxing of the border regime on live TV. Asked when the new rules would come in effect, Schabowski slipped up and inadvertently toppled the Wall a day early by saying “immediately”. Tens of thousands of East Berliners made their way to the border posts, pleading with the surprised border guards to let them through. Heavily outnumbered, the Bornholmerstrasse border opened first at 21:20, with others as well as posts along the main West German border following soon, and thousands of people partied in the streets. The GDR was effectively finished – in December the SED's monopoly position was undone, and two days later Krenz and the politbureau resigned. East and West Germany were unified a year later, and Berlin started its transformation.

Visit the Cold War Berlin page for more information and sights.