Belgrade will be accused at the International Court of Justice of sponsoring ethnic cleansing in the 1990s which led to the worst massacres on European soil since the Second World War. Previously, only individuals have been charged with genocide, the most serious war crime.
Meanwhile, EU foreign ministers in Brussels will threaten to hold up talks on the creation of a free trade zone with Serbia because of Belgrade's failure to arrest the former Bosnian Serb military commander, Ratko Mladic.
This rebuke may not satisfy the UN's chief war crimes prosecutor, Carla Del Ponte, who has called on the EU to put pressure on Belgrade and privately pressed for a formal suspension of the negotiations.
The two events underline the extent to which the region's bloody past overshadows Belgrade's effort to draw closer to the EU.
Today's hearing at the World Court comes 13 years after Bosnia and Herzegovina first charged the Serb authorities, accusing them of the worst case of ethnic cleansing since the Nazis' Final Solution. Though the Serb military was not involved in the war, Belgrade stands accused of backing the Bosnian Serb forces.
Were the case to be proved, the Belgrade could be forced to pay billions of euros in compensation, as well as suffering the disgrace of being labeled a state sponsor of genocide.
Bosnia argues that only by acknowledging Belgrade's role in massacres there can reconciliation be achieved.
Genocide was outlawed under the 1948 Genocide Convention, though the first conviction came 50 years later when a UN court on Rwanda sentenced a former mayor, Jean-Paul Akayesu, to life imprisonment.
Since then, hearings at the UN's International Criminal Tribunal for the Former Yugoslavia in The Hague have decided that genocide took place at Srebrenica. The tribunal has convicted the Bosnian Serb military commanders Radislav Krstic and Vidoje Blagojevic of the offence.
But showing that genocide - a difficult crime to prove - was committed by a state will be more difficult. A paper produced by the Institute for War and Peace Reporting argues that genocide "requires a particular mindset - namely the special intent to destroy a population - which, on the face of it at least, appears difficult to attribute to a state".
It points out that, because Serbia's alleged sponsorship of genocide was through proxies, it will be even harder to prove.
The World Court may decide it does not have legal jurisdiction. A case taken by Serbia against Nato countries was thrown out in 2004 because Yugoslavia was not a member of the UN at the time. Belgrade is likely to argue that, if it cannot initiate a case at the World Court, if cannot be called on to defend one.