The scenes may have been chaotic but last week’s opening of the trial against Charles Taylor, the warlord indicted on 11 counts of war crimes and crimes against humanity in both Liberia and Sierra Leone, marks a key moment. The trial, which Taylor refused to attend, is the first in which an international forum has held a former African president accountable for his conduct while in office.

By coincidence, also last week the chief prosecutor for the International Criminal Court (ICC) told the United Nations Security Council that Ahmad Harun and Ali Kushayb, alleged Darfur war criminals, must be arrested. Luis Moreno Ocampo urged the council: “We count on every state to execute an arrest should either of these individuals enter their territory.” Harun is the Sudanese minister responsible for providing humanitarian assistance to more than four million people in Darfur, while Kushayb is a leader of the Janjawid. Together, they are charged with 51 counts of war crimes and crimes against humanity. In April a panel of three judges at the ICC issued warrants for their arrest.

The pair systematically pursued civilians; Harun organised a system for recruiting, funding and arming the Janjawid to supplement the Sudanese armed forces and then incited them to commit murder, rape and other crimes against the civilian population, the prosecutor argues. The situation in Darfur, he says, “remains alarming”.

The investigation by the office of Moreno Ocampo is the third involving long-running conflicts: the others are those in northern Uganda and the Democratic Republic of Congo. Each involves thousands of killings, as well as “large-scale sexual violence and abductions” and together they have resulted in the displacement, he says, of “more than five million people”.

But the Darfur investigation could lead to the first prosecution by the court since it was set up in 2002, involving a member of an African state. The trial of Taylor is taking place in the ICC’s own court in The Hague, but it is not an ICC case: the trial, being held at a safer venue than Sierre Leone, is being conducted by the UN Special Court for Sierra Leone, set up to deal with the crimes arising from that country’s civil war. Moreno Ocampo says: “The Rome Treaty that set up the court is not just about accountability – bringing people to justice for these massive crimes. It is setting up a global legal system. And it is a back-up, a court of last resort where a state can’t or won’t act. We are exporting the idea around the world that if someone commits genocide, they can be prosecuted for it. That’s why it’s so important. It’s developing a framework, a legal constraint.”

The task has been enormous. The prosecutor, who was elected by all parties to the Rome treaty in 2003, was referred the case by the UN Security Council. “That’s important. It connects a peace and security system with a justice system.” Moreno Ocampo runs the investigation himself. He has no powers to indict – the evidence goes to a panel of ICC judges who decide if charges should be brought. After 20 months the files went to the panel in February. Both men are in Darfur: Kushayb is thought to be in detention but with little prospect of a trial. The prosecutor’s office collected statements from some 100 witnesses after screening 600 over 70 visits in 17 countries. More than 200,000 refugees have fled to other countries. It was important to trace those who had fled, the prosecutor says: “We have a legal duty to protect witnesses and had no chance of doing that in Darfur.”

Moreno Ocampo, 54, has an outstanding track record: between 1984 and 1992 he was a prosecutor in his homeland of Argentina, where he was involved in precedent-setting prosecutions of top military commanders for mass killings and other human rights abuses. He was the assistant prosecutor in the junta trial against army commanders, including three former heads of state – the first case against commanders responsible for mass killings since Nuremberg. Then, in 1992, he set up his private law firm specialising in corruption control for large firms and in criminal and human rights law. His work included acting for the victims in the extradition of the former Nazi officer Erich Priebke to Italy, the trial of the chief of the Chilean secret police and several cases involving political bribery, journalists’ protection and freedom of expression.

In his present role, he emphasises that not all cases will result in a trial hearing. He recently ruled out an investigation into alleged abuses in Iraq, which is not a state party to the treaty; nor did allegations meet the criteria in the Rome treaty; crimes of aggression (the legality of the conflict) falls outside the remit of the ICC and other evidence did not amount to war crimes against humanity. But even if, he says, atrocities are halted, as in Rwanda, then “that is a result: for me it’s the idea that justice can help to prevent crime, even if it does not come to trial”.

In his update to the UN Security Council last week, Moreno Ocampo urged every state to ensure pressure on Sudan to arrest and refer the pair to the ICC. Meanwhile, “indiscriminate and disproportionate” air strikes by the Government there continue. It appears that “parties to the conflict continue to violate international humanitarian law. Those bearing the greatest responsibility must be brought to justice.”

Like Iraq, Sudan is also not signed up to the ICC, but it is a member of the UN Security Council. Moreno Ocampo is hopeful of cooperation. “For the first time we have the UN Security Council and the ICC working together – the first time a new global legal system is tested. It will be a huge step – a legal revolution.”

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