Kosovo has always occupied a special place in Serbian history and hearts despite having a majority of ethnic Albanians who now outnumber local Serbs by about 12 to one. The result of the weekend referendum approving the new Serbian constitution confirms that. Kosovo has existed in a kind of international limbo since Nato's intervention in 1999. Though still technically part of Serbia it has been run (and fairly poorly) as a United Nations protectorate pending a final decision on its future. It was always going to be hard to work out what to do. The latest news from Belgrade may make it harder.

Vojislav Kostunica, Serbia's nationalist prime minister, campaigned for a new constitution for a country that, having lost Montenegro, is all that remains of Yugoslavia and, like Slovenia and Croatia, would like to follow a democratic and reformist path to the EU. Referring to Kosovo as an "inalienable" part of Serbia may reflect broad patriotic feeling but it also panders to radical ultranationalists (led by a man facing war crimes charges in the Hague) - and perhaps help leaders duck responsibility for the province's impending loss.

For Mr Kostunica surely understands that Kosovo's fate was sealed when Slobodan Milosevic went on the offensive in 1998, eventually triggering Nato's attack and his own downfall. But the fear is that the Serbs of Kosovo will take the constitution's pledge at face value - believing it can stop their "amputation" from Serbia. That could be a dangerous illusion that may encourage them to try to force partition, perhaps triggering violence and a mass exodus.

It was clear before the referendum that negotiations being run by Finnish diplomat Martti Ahtisaari for the UN were facing a crunch moment, partly because of old-fashioned great power rivalry. Vladimir Putin has hinted that if the west does push for a Kosovo that is no longer formally part of Serbia, whatever its precise status, he might encourage other pro-Russian regions to seek independence: Trans-Dniester, a province of Moldova, and Abkhazia and South Ossetia, in Georgia.

If Moscow backed Belgrade, the US and Britain might recognise a unilateral declaration of independence by Kosovo. The Kosovars certainly want freedom and sovereignty. Their prime minister, Agim Ceku, describes his country as "the final piece of the European puzzle". But completing it looks hard. Postwar nation-building is not easy either. Those thoughts are reminders that, from the Balkans to Baghdad, western military intervention is invariably only the beginning, not the end of the story.