The only permanent international war crimes court has opened its first hearing, in the case of a Democratic Republic of Congo militia leader.
Lubanga denies three war crimes charges
Judges at the International Criminal Court (ICC) are to decide whether Thomas Lubanga should stand trial for allegedly recruiting child soldiers.
The five-year DR Congo conflict led to an estimated four million deaths.
The US strongly opposed the creation of the ICC, fearing the political prosecution of its soldiers.
The ICC was designed to end the need for the various ad hoc war crimes courts which have recently been established, including the chambers created to deal with war crimes committed in the former Yugoslavia and the genocide in Rwanda.
Mr Lubanga, 45, led the Union of Congolese Patriots (UPC) militia in DR Congo's north-eastern Ituri district, where fighting continued long after the official end of the five-year war in 2003.
"Lubanga made children train to kill, Lubanga made them kill and Lubanga let the children die... in hostilities," prosecution lawyer Ekkehard Withopf told the court.
The prosecution says he visited a training camp for his mostly ethnic Hema forces, which included children as young as 10, preparing to battle their Lendu rivals.
"Whilst encouraging them, they [Mr Lubanga and his deputy] also threatened that they would be killed if they attempted to flee the camp," the prosecution statement says, reports the AFP news agency.
The child soldiers were later instructed "to kill all Lendu including men, women and children", the statement says, based on testimony from six children.
He denies three charges of war crimes.
His lawyers say he was trying to end the conflict and is being punished by the international community for refusing to give mining concessions in areas he controlled to foreign firms.
Referring to his enemies, he once told UN peacekeepers: "Those who have committed genocide or massacres have to be punished."
The BBC's Mark Doyle says the conflict in Ituri manifests itself as an ethnic war, but its root cause is the criminal mining of the region's gold and other minerals.
Mr Lubanga's pre-trial is set to last three weeks. The judges will then decide whether to take the case to trial.
The pressure group Human Rights Watch (HRW) has welcomed the first hearings of the ICC, but it says the case against Mr Lubanga is far too narrow in its scope.
"This is very important both for international justice and justice for the Congolese people," HRW researcher Anneke Van Woudenberg told the BBC's Network Africa programme.
HRW says some 60,000 civilians have been slaughtered in Ituri province by various rebel groups, and that they should all be investigated, as should government officials from DR Congo and others who may be implicated from neighbouring Rwanda and Uganda.
Despite US opposition to the ICC, it did not object when the United Nations Security Council referred atrocities committed in Sudan's Darfur there.