Can the Balkans finally turn the page? Ratko Mladic, the Serb general who was Milosevic's military spear-carrier in the long wars of anti-Muslim extermination and ethnic cleansing in the western Balkans in the 1990s, has finally been caught. Can Belgrade use this achievement to face down its nationalist past and finally bring closure on the Kosovo question, which is blocking a future for Serbia in the EU?

Like some wretched SS general hiding away after 1945, Mladic had a network of supporters and admirers who protected him from justice. He continued to receive his army pension – paid to his family – until a few years ago. Too many Serbs have been in denial about his role, too ready to forgive him as they woke up ten years ago to discover that Milosevic, Mladic and Karadzic had finally buried the major European federal state that Tito created after 1945.

Serbia's moderninising, pro-European president, Boris Tadic, a committed social democrat, always insisted both as defence minister and then as president that he wanted to bring Mladic to trial. Now he has delivered. As Balkans minister from 2001-2005 and then as UK delegate to the Council of Europe, I insisted that Serbia had to deliver – not protect – Mladic, if the door to Europe was to open fully. Now Tadic deserves Europe's thanks.

But will he snatch diplomatic defeat from the jaws of the Mladic arrest victory? In one of those spectacular coincidences that Balkan history adores, Tadic was due to meet President Obama tomorrow in Warsaw. But, in a misguided diplomatic blunder, Tadic declared earlier this week that he would snub the US president by refusing to go to Poland to greet him.

The reason for this was that Kosovo was invited to take part in the meeting along with the other ex-Yugoslav states. Yet Tadic could still turn up in Warsaw as Europe's hero of the hour, having finally hunted down the western world's most wanted war criminal to send him to The Hague for trial.

Instead the official Serb line – at least until the news of Mladic's arrest – was that Belgrade was so offended by the presence of Kosovo's president in Warsaw that Tadic would not fly to Warsaw to shake Obama's hand.

Coup in Poland

Washington is still strongly committed to supporting the new post-communist democracies of eastern Europe and the Balkans. While George W Bush took his eye off the Balkans to focus on Iraq and Afghanistan, Barack Obama will not forget that it was the US that helped defeat the Milosevic murder machine, first in Bosnia and then in Kosovo.

Obama's key foreign policy advisers, as well as Vice-President Joe Biden and the secretary of state, Hillary Clinton, are children of the 1990s who watched in frustration as neither the UN nor the EU was able to stop neither the genocidal slaughter in Srebrenica nor the mass killings of Kosovar Muslims by Serb paramilitaries under "generals" such as Ratko Mladic.

Getting Obama to Warsaw is a remarkable coup for Radek Siksorski, Poland's single-minded, outspoken but strategic foreign minister. Oxford-educated Sikorksi has moved on from a 1980s neoconservatism to his position today as Europe's most innovative and creative foreign minister. He has helped reduce the age-old Poland enmity of its two neighbours, Russia and Germany, to manageable, even friendly, relationships.

He has more contacts in Washington than any other European politician and has parlayed these into Obama's major visit. To place Poland as the pivot of Euroatlantic politics in the new Europe, Sikorski invited all European nations to come and talk with Obama.

These now include the new nation state of Kosovo, which is recognised by 75 other countries and is accepted as a partner by international financial institutions. But Kosovo faces a determined campaign by Serbia to deny the young nation's identity. Russia, which never loses an opportunity to meddle, backs Belgrade's anti-Kosovo line. Serb anti-Kosovar diplomacy is the continuation of Milosevic's war by other means.

Demonic obsessions

Serbia has allies in a few EU member states, notably the two Orthodox countries of Greece and Cyprus. Spain has also broken ranks with EU solidarity, citing the flimsy excuse that recognising Kosovo would encourage Catalan and Basque separatism – as if recognising other ex-Yugoslav entities would not.

Slovakia and Romania are rightly angry at the nationalist populism of Hungary's new conservative government, which issues passports to Hungarian minorities in its two neighbours, with the clear implication that the Hungarian-origin citizens of Slovakia and Romania owe first loyalty to Budapest.

Only Romania has decided to make its non-recognition of Kosovo a reason to reject Sikorski's invitation to Warsaw to sup with Obama. Both the US and Sikorski say firmly that the Kosovar president, Atifete Jahiga, will attend the Warsaw summit.

Sikorski was very clear. "Kosovo is recognised by some 75 countries, including the majority of the European Union. Poland recognises Kosovo so there was no reason not to invite Kosovo's representative," he told the Financial Times. He also had a blunt message for President Tadic. "Serbia has to show that it has overcome the demons from its past if it wants to join the European Union."

In fact, the EU's foreign affairs chief, Cathy Ashton, has been quietly but effectively trying to bring Belgrade and Pristina together. Her chief aide, the able British diplomatist Robert Cooper, has been involved in quasi-shuttle diplomacy between Brussels, Belgrade and Pristina.

Smart Serbs know that the notion that Kosovo is just a breakaway province which will accept rule from Belgrade is a fiction. Sadly, some EU member states such as Spain, Greece, Slovakia and Romania help Belgrade – and, behind Belgrade, Moscow – maintain this fiction.

Obama and Sikorski have called this bluff by insisting that Kosovo has much right as any EU nation to sit next to representatives of other Balkan states at international conferences. Serbia is locked in denial on this issue.

The question now is whether the arrest of Mladic allows Tadic to come triumphantly to Warsaw, or whether Serbian nationalist irredentism insists he stay in Belgrade rather than sit at the same table as his fellow-president from Kosovo. The sooner all of the EU, and the rest of the world, let Kosovo be Kosovo, the quicker Serbia can start its push for EU membership.

Mladic's arrest should be the moment when Serbia cuts its Kosovar Gordian knot and moves forward to Europe, rather than remembering the past when Serb bullies such as Mladic assumed that they could rule other peoples and nations demanding freedom.

Denis MacShane is a British MP and a former minister for Europe