The German question would remain open as long as the Brandenburg Gate was closed, West Germans used to say. And then suddenly, twenty years ago, that gate was unexpectedly forced open — and the division that since the end of the Third Reich had cut Berlin, Germany and Europe in two collapsed. The Wall itself did not collapse: it was hacked to tiny pieces by thousands of incredulous Berliners. They chipped off the graffiti, opened huge holes in the reinforced concrete barrier and mocked the bemused East German police who stood around, unsure whether to shoot, flee or rejoice.

No one who was there will ever forget the extraordinary weekend that followed the rush to the Wall, the jostling of the Stasi guards, and the lifting of the barriers at the crossing-points. As the tottering East German regime imploded in demonstrations, bankruptcy and farce, its frustrated citizens streamed across in search of bananas, relatives and freedom. Nothing could rival the amazing television pictures. All who watched knew that this was history, spontaneous, chaotic and irreversible, in the making.

Today European leaders gather in a peaceful, united Berlin to commemorate the event, and to remind their continent that this was one of the few great convulsions of European history that happened almost bloodlessly. Tomorrow some of its architects, including Mikhail Gorbachev and Lech Walesa, the Soviet President and Polish opposition leader, will describe their roles in the chain of events that earned them Nobel peace prizes. By good fortune almost nothing went wrong. Moscow did not send in troops. The enfeebled communist leaders of eastern Europe did not — apart from the despicable Ceausescu — turn their guns on their people. Demonstrations did not turn into riots. German unification was negotiated between the wartime allies and their former enemy carefully and successfully, despite the doubts of Margaret Thatcher and President Mitterrand.

Unification has come at a price, however. Few in Bonn realised how costly would be the ruinous decision to exchange the East German mark at parity. No one in the West knew the scale of the country's foreign debt and industrial ruin, how hollow were its boasts and how polluted its landscape. It has taken two decades, the flight of three million people from the East and untold investment to try to repair 40 years of communist misrule. Even now eastern Germany is significantly poorer, with higher unemployment, than the west. But it has freedom. And who would have imagined then that the Chancellor of a confident and stable Western democracy celebrating this moment would be the daughter of an East German pastor?

But just as the Wall had become the overused symbol of a corrupt system that denied opportunity, liberty and justice, so its destruction was hailed as the precursor of prosperity across half a continent. That is not what happened. The end of communism brought also the collapse of stability, the unleashing of ancient hatreds, the unrealisable hopes of peoples unprepared for the 20th century and an explosion of crime and corruption. In much of the Balkans, politics reverted to the nationalisms before the First World War. Integrating the East into Nato and the European Union was a noble ideal. But it has been a hard and expensive task. There still remains a gulf between East and West.

European leaders in Berlin must look today at what has not yet been achieved. They must commit themselves to real political liberty, to compassionate capitalism and to a social tolerance compatible with European ideals. It is easy to see why the Wall had to go. It is not so easy to see the contours of the new freedoms.