How hard can it be to arrest a man recognised wherever he goes, in a country roughly the size of Scotland? Vojislav Kostunica, the Serbian Prime Minister, has asked the international community to believe that if the man in question does not want to be found, the answer is, effectively, “impossible”.

This answer is not acceptable. It is entirely appropriate that Belgrade’s failure to surrender Ratko Mladic to the UN war crimes tribunal as promised led yesterday to the suspension of talks with the EU on possible Serbian membership. Mr Kostunica claimed that his Government had done its utmost to seize Mr Mladic, the alleged architect of the 1995 massacre of some 8,000 Muslims in Srebrenica.

But the EU was right to call his bluff, both in principle and from the point of view of what should have been possible: The Prime Minister’s own deputy promptly resigned, accusing Serbia’s security services of seeking Mr Mladic “everywhere except where he was hiding”. Wherever that is, the former general and his diehard loyalists now have the satisfaction of knowing they have jeopardised what international standing Serbia has earned since surrendering Slobodan Milosevic to The Hague, weakened Mr Kostunica’s minority coalition and postponed indefinitely the start of Serbian EU accession talks — which polls indicate 60 per cent of Serbs want.

The challenge facing Mr Kostunica should not be underestimated. He is dependent on the support of Socialists still loyal to the memory of Mr Milo-sevic. He is also constantly threatened by the spectre of resurgent nationalism. Surrendering Mr Mladic is not the only test of Belgrade’s good intentions, but, like the imminent prospect of independence for Montenegro and, possibly, Kosovo, it is guaranteed to inflame that nationalist tendency.

This is why the decision taken yesterday by Olli Rehn, the EU Enlargement Commissioner, was never going to be simple. It was nonetheless correct. Unlike Croatia in comparable circumstances last year, Serbia has produced little hard evidence of genuine effort to track down or isolate those of its indicted war crime suspects who remain at large. Yet it seems clear that the security services know where they are. Meanwhile, insisting on Mr Mladic’s surrender is by far the EU’s most powerful lever in its dealings with Belgrade. To surrender that lever would be to surrender what credibility the enlargement process retains in the wearier capitals of “Old Europe”.

What should Mr Rehn do next? Rather than shatter progressive Serbians’ hopes of eventual EU membership, he should keep that hope explicitly alive, subject to full co-operation in the hunt for Mr Mladic and a mature acceptance by Belgrade of Montenegrin independence should the referendum due this month demand it. Above all, he must persuade the foot-draggers in Paris and Berlin that enlargement remains the EU’s noblest cause. Steady progress towards membership for Croatia, Montenegro, Kosovo and then Serbia itself is the best long-term prescription for Balkan prosperity and, yes, peace.