Yesterday morning Mikhail Saakhashvili, the Georgian President, gave warning that, if reports of Russian armour entering his country were true, it would mean war. They were true. Tanks were crossing the international border from Russia into the breakaway Georgian province of South Ossietia as Mr Saakashvili spoke.

He also promised a brief ceasefire to allow the evacuation of wounded civilians. Such ceasefires may come and go. The war may remain officially undeclared. But Russia and its most fervently pro-Western neighbour are now locked in open military conflict.

So far the violence has been contained within an area smaller than Kent in the remote southern Caucasus. But a long-frozen conflict on the fringe of the old Soviet empire has unfrozen with alarming speed and literally incalculable significance. Regional stability, the future of European energy supplies and the tone of Russia's already fraught relations with the West are all at stake.

Vladimir Putin, in Beijing for the opening ceremony of the Olympics, blamed the escalating crisis on Georgia but insisted that “nobody wants to see a war”. If so, peace is still attainable, but only if the international community can persuade reactionary voices in both Moscow and Tbilisi - Mr Putin's included - to abandon their self-defeating rhetoric.

The immediate origins of this crisis lie in a pledge by Mr Saakashvili, as he contested the Georgian presidency in 2004, to win back direct control of South Ossetia and nearby Abkhazia. Both provinces lie within Georgia's internationally recognised borders. Both fought unsuccessful wars for independence from Georgia in the 1990s. Both still seek it. Neither is officially recognised as independent by any other country, but under Mr Putin Russia openly supported their separatist factions and distributed Russian passports en masse to their citizens, whom the troops pouring south through the Roki tunnel yesterday claimed to be protecting.

To Russia's neighbours and its former enemies the narrative is both alarming and familiar - a fact not lost on Mr Saakashvili. He likened the arrival of Russian military hardware on Georgian territory yesterday to the Soviet invasions of Afghanistan in 1978 and Czechoslovakia in 1968. In a direct appeal to the US and Nato (via CNN) he then added: “It's not about Georgia any more. It's about America, its values.”

It is true that Georgia is now broadly democratic; true, too, that the Harvard-educated Mr Saakashvili is fiercely pro-American. But this conflict is less “about America” than about two fundamentally opposed views of the break-up of the Soviet Union.

Western optimists saw in that geopolitical earthquake the creation of 14 new independent countries around a defeated Russia. Mr Putin and the security apparatus that he has installed see it as a catastrophe to be methodically reversed by continuing to treat neighbouring territories as a sphere of influence, and Russian foreign policy there as a zero-sum game in which Western gains must be Russian losses.

In this context, Nato's pledges of allegiance to Ukraine as well as Georgia may seem outrageous. But this does not justify Russia's narrow view of its “near abroad”, or Russian tank columns in South Ossetia. Equally, Western interests in Georgia are based as much on its oil pipelines as its politics. But this does not devalue its sovereignty.

Moscow has explicitly recognised Georgia's territorial integrity. Mr Saakashvili, in his calmer moments, has made serious offers to South Ossetia of autonomy within Georgia. This must be the basis of urgent diplomatic efforts to pull both sides back from the brink.