It is rare that a historical era can be defined with precision, but the fall of the Berlin Wall 20 years ago this week is just such a rarity. The breaching of that monstrosity marked the end of the Soviet empire and ushered in a era of both hope and considerable confusion. There is no mistaking the extraordinary significance of Nov. 9, 1989, but that vista is also a testimony to human failure. The world has been unable to construct a new vision and a new order. It is damning, indeed, that we continue to inhabit "the post-Cold War world."

The Berlin Wall appeared without warning Aug. 13, 1961. It was built by a communist government that ruled East Germany with violence, paranoia and repression. The wall was designed to halt the flow of migrants from East to West, a surge of humanity that sought to flee the Iron Curtain via a bolt hole hundreds of kilometers from the West. Eventually, the wall stretched nearly 150 km around the entire city of West Berlin, a concrete monstrosity dotted with barbed wire, 302 armed-guard towers, and numerous other means to keep East Berliners inside. While West Berlin became a symbol of freedom and hope, the wall, by contrast, was an equally powerful symbol of evil and fear, a symbol that was periodically bathed in blood: It is estimated that some 5,000 people tried to cross the wall; at least 136 people were killed in the effort.

But on Nov. 9, 1989, the symbol came crashing down. Following the opening of other Eastern European borders to the West — Hungary had taken that step in August and Czechoslovakia soon followed — the East German Politburo agreed to open its borders as well. The Politburo spokesperson had not been briefed on the decision and, when asked about details at a late-night news conference, said the order was effective "immediately." Berliners took him at his word and surged to the barrier. Thousands crossed both ways and border guards kept their weapons holstered. "Mauerspechte" (wall woodpeckers) took sledgehammers to it; the East German police tried halfheartedly to fix the damage, but on June 13, 1990, official efforts began to dismantle the barrier. German unification was concluded Oct. 3, 1990.

Two men are generally credited with the fall of the wall. The first was former U.S. President Ronald Reagan, who two years earlier used it as the backdrop for a speech exhorting Soviet President Mikhail Gorbachev "to tear down this wall." His refusal to accept detente with the Soviet Union galvanized international opinion and shifted the terms of engagement between East and West. While it is fashionable to deride Reagan as a foreign policy simpleton, his single-minded devotion to the principles of freedom and democracy was crucial to ending the Soviet Union.

The greatest hero, however, was Mr. Gorbachev. He recognized that the Soviet Union was rotting from within and that its renewal could only begin with the jettisoning of its empire and the acknowledgment of basic human rights of freedom and dignity. He refused to support the East European regimes that relied on Soviet brute force to survive. For his vision, he was rewarded with a coup at home and ignominy among Russians nostalgic for Great Power status.

In Germany, however, he is a hero. Mr. Gorbachev was a guest of honor at the ceremonies marking the 20th anniversary of the collapse of the wall. He was hosted by German Chancellor Angela Merkel, a former East German chemistry researcher, whose own political trajectory began that day.

Others, of course, also played key roles. There were East European democracy leaders who fought Soviet repression and pressed for human rights. There were Western leaders who supported the German drive for unification (sometimes against allies who feared the prospect of a united Germany). And there were millions of Germans and Europeans who recognized the significance of the moment and welcomed a reunited Germany.

Forging that nation was not easy. Despite having spent more than $1.5 trillion, there remain two Germanys with unequal living standards and opportunities. Some Europeans still fear the behemoth at the heart of their continent, but Germany has been a good citizen and its actions are allaying those concerns.

The sense of accomplishment over these 20 years is real. The world has truly changed. But if the structure of the international system is different — the Cold War superpower standoff has ended — no one has created a vision to replace it. We continue to flail in the effort to create a new world order that gives meaning and substance to the drive for human dignity and justice that eroded and eventually collapsed the Berlin Wall. We have failed states, terrorism and rising levels of poverty. There is a seeming inability to tackle the pressing issues of the day and the future.

The fall of the Berlin Wall is a reminder that this description need not remain the reality, that change is possible if we make it so. Human beings make history. It is time to get to work again.