The narrow win of Boris Tadic in Serbia's presidential election comes as a huge relief to European leaders. Had his nationalist opponent convinced the angry electorate that the West had betrayed Serbia and that Belgrade should look instead to Russia to stop Kosovo declaring independence, the tensions in the Balkans could have erupted swiftly into violence. European leaders were therefore quick to congratulate Serbia on its “prudence and far-sightedness” while underlining the importance of swift moves by the European Union to strengthen ties with Belgrade.

Relief, however, must be tempered with realism: the Balkan crisis is far from over. Serbia remains a frustrated and deeply divided nation, and those who supported Mr Tadic - very largely the younger voters - did so because they want to end their country's isolation, not because they have dropped their opposition to Kosovan independence. The President's victory will, if anything, widen the split within the Government, where his Democratic Party rules in uneasy coalition with Vojislav Kostunica, the Prime Minister, who takes a tougher line on Kosovo and has pointedly refused to back Mr Tadic in his dealings with the EU. Instead, Mr Kostunica has blocked the signing of the agreement offered to Serbia on political dialogue, free trade, visa liberalisation and educational co-operation. This is due to be signed on Thursday, but the nationalists will veto the deal unless Brussels renounces plans to send an EU mission to Kosovo, a move that could bring down the Government.

Mr Tadic will also face a further challenge to his careful attempts to hold nationalist emotion in check when, as promised, Kosovo declares its full independence in the next few days. His determination to maintain a dialogue with Brussels rather than appeal to Moscow for political and even military aid shows a realistic understanding of where Serbia's long-term interests lie. It is a line that is deeply resented by many, especially an older generation that fell under the spell of Milosevic's malign mixture of aggressive nationalism and the myth of Serb victimhood.

Europe must therefore move swiftly and visibly to assuage Serb grievances. Removing the obstacles that hamper growth and regional integration is a start: visa restrictions, trade barriers and tariffs. The EU should go farther, and make clear that it is ready to start talks on eventual Serbian membership and that, meanwhile, it will take seriously Belgrade's demand to be consulted and included in regular political dialogue. Until now the EU has insisted that Serbia must first capture and hand over for trial the two notorious Bosnian war criminals still at large, Radovan Karadic and Ratko Mladic. That demand must not be dropped: international justice insists that those responsible for thousands of murders should be brought before the court in The Hague.

The presidential election was hard fought. What Serbia now needs, by contrast, is stability, investment, pragmatic development and outside encouragement. Russia, so far, has accepted the defeat of its chief supporter with restraint. David Miliband, an advocate of EU enlargement, should book a flight to Belgrade to bolster Mr Tadic's victory with support, engagement and the promise of Western backing for his realism.