Richard Holbrooke, the former American Ambassador to the UN, remarked yesterday: “One of the worst men in the world, the Osama bin Laden of Europe, has finally been captured.” As a description of Radovan Karadzic, that is no hyperbole.

Between the declaration of independence in 1992 and the Dayton peace accord that Mr Holbrooke brokered in 1995, Bosnia suffered a campaign of xenophobic savagery by Bosnian Serb forces. Thousands of women were raped. Camps at Omarska, Susica and elsewhere became bywords for beatings, torture and murder. Some 200,000 people were killed, most of them Muslims. In the most horrific act of this war of aggression, 8,000 Muslim men and boys were massacred at Srebrenica.

The agents of this catastrophe were led by Dr Karadzic, and were under the military command of Ratko Mladic. For 13 years, these men have escaped capture. The evil that men do lives after them, but the arrest of Dr Karadzic is a moment of catharsis. It brings nearer a belated historical accounting for appalling crimes. It advances Serbia's place in the family of European nations. It is a notable moment for the values on which Europe is founded, and which were so traduced by the Bosnian Serbs and their Svengali, the late Slobodan Milosevic. Boris Tadic, the Serb President, deserves praise for the arrest of Dr Karadzic (see page 6).

There can be little doubt that one factor that emboldened Mr Tadic in withstanding domestic opposition was the stand of the European Union. After the fall of Milosevic in 2000, Serb leaders worked for the international rehabilitation of the country. The EU refused to allow such rehabilitation while Dr Karadzic and Mr Mladic remained at large. Now some progress has been made.

In April, the European Union signed with Serbia a stabilisation and association agreement, enhancing trade and political links. But progress towards further integration into the EU depends on Belgrade's co-operation with the requirements of the International Criminal Tribunal for the former Yugoslavia (ICTY). The ICTY requires that Serbia arrest war crimes suspects, most notoriously Dr Karadzic and Mr Mladic.

The EU must strike a delicate balance between being seen to encourage Belgrade for apprehending the former, while insisting that the latter join him in the dock. Mr Mladic's ability to evade arrest has been a constant embarrassment to Nato. There are credible reports that he has been given sanctuary at Serb military bases. It is a minimal requirement that Serbia deliver him up to international justice before it becomes a candidate country for EU membership. But it is important that the EU and its member states indicate to Serbia both the benefits of compliance with the ICTY's orders and the costs of failing to do so.

While the capture of Dr Karadzic is an historic moment, trying him at The Hague will not be a straightforward task. The system of trying war crimes is itself on trial. A chance to achieve justice has already been lost through the death, while on trial, of Milosevic, who clandestinely armed the Bosnian Serbs. The ICTY itself is due to be wound up this year, though the chief prosecutor is able to ask the UN Security Council for an extension. The increasingly obstructive diplomacy of Serbia's ally, Russia, is likely to impede a successful prosecution of Mr Mladic if his arrest comes after the deadline.

In this imbroglio, the British Government and the EU must impart a clear message. The Bosnian war was not an explosion of ancient antagonisms. It was a deliberate and meticulously planned campaign of genocidal violence, on the same continent as, and only half a century after, the crimes of Nazism. This is not victors' justice: it is justice long thwarted and at last achievable.