In weeks and years past, each of us has argued that Moscow was pursuing a policy of regime change toward Georgia and its pro-Western, democratically elected president, Mikhail Saakashvili. We predicted that, absent strong and unified Western diplomatic involvement, we were headed toward a war.

Now, tragically, an escalation of violence in South Ossetia has culminated in a full-scale Russian invasion of Georgia. The West, and especially America, could have prevented this war. We have arrived at a watershed moment in the West’s post-Cold War relations with Russia.

Exactly what happened in South Ossetia last week is unclear. Each side will argue its own version. But we know, without doubt, that Georgia was responding to repeated provocative attacks by South Ossetian separatists controlled and funded by Moscow. This is a not a war Georgia wanted; it believed that it was slowly gaining ground in South Ossetia through a strategy of soft power.

Whatever mistakes Tbilisi has made, they cannot justify Russia’s actions. Moscow has invaded a neighbor, an illegal act of aggression that violates the United Nations Charter and fundamental principles of cooperation and security in Europe. Beginning a well-planned war, including cyber-warfare, as the Olympics were opening violates the ancient tradition of a truce to conflict during the Games.

And Russia’s willingness to create a war zone 25 miles from the Black Sea city of Sochi, where it is to host the Winter Games in 2014, hardly demonstrates its commitment to Olympic ideals. In contrast, Moscow’s timing suggests that Prime Minister Putin seeks to overthrow Mr. Saakashvili well ahead of our elections, and thus avoid beginning relations with the next president on an overtly confrontational note.

Russia’s goal is not simply, as it claims, restoring the status quo in South Ossetia. It wants regime change in Georgia. It has opened a second front in the other disputed Georgian territory, Abkhazia, just south of Sochi. But its greatest goal is to replace Mr. Saakashvili — a man Mr. Putin despises — with a president who would be more subject to Moscow’s influence.

As Sweden’s foreign minister, Carl Bildt, pointed out Saturday, Moscow’s rationale for invading has parallels to the darkest chapters of Europe’s history. Having issued passports to tens of thousands of Abkhazians and South Ossetians, Moscow now claims it must intervene to protect them — a tactic reminiscent of one used by Nazi Germany at the start of World War II.

Moscow seeks to roll back democratic breakthroughs on its borders, to destroy any chance of further NATO or European Union enlargement and to reestablish a sphere of hegemony over its neighbors. By trying to destroy a democratic, pro-Western Georgia, Moscow is sending a message that, in its part of the world, being close to Washington and the West does not pay.

This moment could well mark the end of an era in Europe during which realpolitik and spheres of influence were supposed to be replaced by new cooperative norms and a country’s right to choose its own path.

Hopes for a more Democratic Russia under President Medvedev will need to be reexamined. His justification for this invasion reads more like Brezhnev than Gorbachev. While no one wants a return to Cold War-style confrontation, Moscow’s behavior poses a direct challenge to European and international order.

What can we do? First, Georgia deserves our solidarity and support. Georgia has supported us; its more than 2,000 troops are the third-largest contingent in Iraq — understandably those troops are being recalled. We must get the fighting stopped and preserve Georgia’s territorial integrity within its current international border. As soon as hostilities cease, there should be a major, coordinated transatlantic effort to help Tbilisi rebuild and recover.

Second, we should not pretend that Russia is a neutral peacekeeper in conflicts on its borders. Russia is part of the problem, not the solution. For too long, Moscow has used existing international mandates to pursue neo-imperial policies. We must disavow these mandates and insist on truly neutral international forces, under the United Nations, to monitor a future cease-fire and to mediate.

Third, we need to counter Russian pressure on its neighbors, especially Ukraine — most likely the next target in Moscow’s efforts to create a new sphere of hegemony. America and the European Union must be clear that Ukraine and Georgia will not be condemned to some kind of gray zone.

Finally, America and the European Union must make clear that this kind of aggression will affect our relations and Russia’s standing in the West. While Western military intervention in Georgia is out of the question — and no one wants a 21st-century version of the Cold War — Moscow’s actions cannot be ignored.

There is a vast array of political, economic, and other areas in which Russia’s role and standing will have to be reexamined. Moscow must also be put on notice that its own prestige project — the Sochi Olympics — will be affected by its behavior.

Weak Western diplomacy and lack of transatlantic unity failed to prevent an avoidable war. Only strong transatlantic unity can stop this war and begin to repair the immense damage done. Otherwise, we can add one more issue to the growing list of this administration’s foreign policy failures.

Mr. Asmus, a deputy assistant secretary of state in the Clinton administration, is executive director of the Brussels-based Transatlantic Center of the German Marshall Fund of the United States. Mr. Holbrooke, U.S. ambassador to the United Nations in the Clinton administration, writes a monthly column for the Washington Post.

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