Yesterday brought a further rise in tension over South Ossetia, the province of Georgia that wants to use support from Russia to break away. Nato, while saying that it had not discerned a build-up of Georgian troops on the border, called on all parties to try to bring calm.

This week South Ossetian authorities said that they had started to put children on buses to take them out of potential conflict, according to reports. Israel said yesterday that it would stop selling arms to Ossetia to avoid annoying Russia, a move thought to be inspired by a desire for Russian help over Iran. And Russia said yesterday that it “will not allow itself to remain indifferent” if violence escalated, not least because of the large number of Russians living there.

This long-simmering conflict is getting much worse and European diplomats fear that it could even trigger war. It already has the makings of one of the coming year’s worst disputes between Russia and its neighbours, one that could easily turn Russia against Europe and the US.

Why now? The main reason is Georgia’s desire to throw in its lot with Nato, the US’s enthusiastic support for that, and Russia’s passionate opposition. At the Nato summit in Bucharest in the spring, President Bush made one of the stronger speeches of his tenure, arguing that the 26-member alliance should take in Georgia and Ukraine. Germany (strongly) and France (more quietly) argued against this. Angela Merkel, the German Chancellor, said that it would annoy Russia too much to justify the expansion (also throwing in some brisk assertions about the two countries not being ready). The meeting pacified Russia by denying the two a “membership action plan” – a scheme for moving towards that goal – but promised them eventual membership (without mentioning a date).

That took the heat out of the dispute for a month or two but Nato will look again at the question in December, and may well decide to set them on the road to membership.

That is the point when, Western diplomats believe, the clash could occur. Russia might respond by recognising the two separatist provinces, South Ossetia and Abkhazia (which declared independence in 1999, but which Tbilisi regards as a breakaway region). Nato governments, alarmed at the possibility of new “Northern Cypruses” – regions recognised only by their patrons – have hardly begun to think through how they would respond in turn.

Worry about that mess may put Nato members off the decision in December. So may the need for Russian help on other fronts; Israel is not the only country wondering how to persuade Moscow to help to bring pressure on Iran.

Russia is increasingly self-confident abroad, buoyed by high oil and gas prices, and apparently unconcerned about courting support in Europe and the US. There are few deals other countries can strike with the Kremlin, and Georgia has become one of the most valuable bargaining chips around.

All the same, offering membership is not a move that Nato can postpone for ever, given the promises already made. Rebuffing Georgia, where polls show solid public enthusiasm for joining Nato, would carry its own huge cost.

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