By launching its first invasion of a foreign country since the fall of the Soviet Union, Russia jeopardised its status as a rational actor on the world stage. Yesterday it salvaged that status.

The ceasefire announced by President Medvedev after five days of fighting in South Ossetia will help to avert a full-scale humanitarian disaster in the Caucasus. It has created a breathing space for peace talks now under way in Moscow. It leaves President Saakashvili of Georgia humbled internationally even if he remains popular at home, and Moscow confident that its hegemony in South Ossetia and nearby Abkhazia will not be challenged again for the foreseeable future. From the Kremlin's standpoint, it does more: Mr Medvedev and his implacable Prime Minister, Vladimir Putin, can now claim to their more credulous supporters that in the struggle for Georgia's borderlands they hold the moral high ground.

They do not. It is true that Mr Medvedev halted Russia's troops before they could mount a full-scale advance on the Georgian capital and without hinting at regime change there as one of the Kremlin's goals. Insisting only on a new non-aggression pact and the withdrawal of Georgian troops from the two disputed enclaves, he was able to present himself as the voice of emollient reason.

Mr Saakashvili has little choice but to accept Russia's terms. His dream of a reunited Georgia now looks as tattered as his army. Georgian voters may yet punish him for trusting naively in the traditional Olympic fortnight truce and an informal alliance with the US to deter Russia's military might.

There is no question that Mr Saakashvili has disastrously overplayed his hand; nor that France and Germany are now likely to argue ever more forcefully against Georgia's other dream of Nato membership, for fear of further provoking Moscow. But none of this changes the basic narrative of the past five days, or its lessons for the West.

Georgia is a sovereign, independent country. Russia invaded it, ostensibly to protect the people of South Ossetia but in reality to punish Tbilisi for seeking membership of Nato, and the alliance for offering it; to highlight what it considers the West's double standards in supporting self-determination for Kosovo but not in the Caucasus; and to destabilise Mr Saakashvili (whom Mr Putin, not incidentally, despises).

The message for Russia's former Soviet satellites is that Moscow still seeks their subservience. That message will have been heard loud and clear from Riga to Tashkent. The response from Western governments must be equally clear: Russia cannot be allowed to dictate its neighbours' alliances, nor their forms of governance. If a broadly democratic Ukraine seeks EU and Nato membership it should be allowed to pursue them without fear of Soviet-style retaliation. For all Mr Saakashvili's flaws, the same is true of Georgia.

Jaap de Hoop Scheffer, the Nato Secretary-General, was therefore right yesterday to restate Nato's pledge of eventual membership for Georgia, and to insist that Russian troops not merely halt but pull back to positions sanctioned by existing peacekeeping agreements.

Nato was conceived to offer the US and its European allies collective security in the face of an existential Soviet nuclear threat. With Soviet ideology dead, not even Mr Putin can rekindle that threat. In the long term Nato therefore needs to clarify its mission to reassure post-Soviet Russia. At the same time, Russia needs to modernise its foreign policy to embrace political diversity along its borders. The irony is that Nato must first do what it was originally designed for and tell Moscow: this far, and no farther.