Fall of the Wall
While the date "9/11" will reverberate in our memories forever, the date "11/9" is equally significant. It was on November 9th, 1989, that the East German government (acceding to citizen demands after several weeks of unrest) allowed free transit between socialist East Berlin and democratic West Berlin.
This decision made the Berlin Wall moot, with souvenir-seekers chipping away pieces of the wall until heavy equipment could remove it. It also led to the formal reunification of the Deutsche Demokratische Republik (East Germany) and the Bundesrepublik Deutschland (West Germany) less than a year later.
Most significantly, this decision brought the Cold War to an end -- ending more than four decades of tension and anxiety between the world's two dominant nuclear superpowers.