If you want to understand this year's expansion of the European Union and NATO, go to Berlin.
Fifteen years ago, the Berlin Wall, approximately 96 miles of concrete and soldiers, was the symbol of the Cold War. When I visited this spring, my taxi driver had to tell me to "imagine" where the wall had stood. Today, the new building in the center of Berlin means you literally need a historical map to find its path.
In some ways, the reunification of Berlin, and of Germany, is a special case. It was political and economic integration within one country. And it happened quickly after the fall of communism.
But the metaphor of two returning to one is nearly perfect for the wider European story: Western Europe and Eastern Europe are once again simply Europe.