Guilty: Congolese warlord who built an army of children...

...but why has the International Criminal Court's first conviction taken six years?

After six years of hearings and £600m of funding, the International Criminal Court delivered its first conviction yesterday, finding the former rebel leader Thomas Lubanga guilty of war crimes in the Democratic Republic of Congo.

The court's supporters hailed the ruling as a landmark for international justice while its critics pointed to the slow pace of prosecution and mounting costs as evidence of its failure.

The first person to be arrested by the ICC will now await sentencing after the judges found overwhelming evidence that he recruited child soldiers and sent them into battle in north-eastern Congo. The conflict left as many as 50,000 people dead in the Ituri region alone, and forced many more to flee their homes.

The warlord's forces have been accused of carrying out ethnic massacres, torture, rapes and forcing young girls into sexual slavery. But he was convicted of the more limited charges of "conscripting and enlisting children under the age of 15 and using them to participate actively in hostilities".

Lubanga, who at one point had 3,000 child soldiers under his command, led the Union of Congolese Patriots in an ethnic war that devastated the gold mining area close to the border with Uganda. Human rights workers have accused several international mining firms of colluding with armed groups in the area.

The once-feared warlord, who taxed families for their cattle and children, has gone through a marathon trial that often looked likely to collapse due to its complexity. In 2008, judges ordered him to be released on a technicality but were persuaded to keep him in custody while prosecutors appealed. "Today's verdict will give pause to those around the world who commit the horrific crime of using and abusing children both on and off the battlefield," said Amnesty International's Michael Bochenek. The rights watchdog is among several groups who have attacked the limited scope of the charges brought in the Lubanga case, complaining that allegations of sexual violence against abducted girls were not pursued.

The ICC is the world's first permanent court set up by an international treaty ratified in 120 countries to deal with war crimes, crimes against humanity and genocide. It followed on from a series of ad hoc tribunals set up to deal with the aftermath of conflicts in the former Yugoslavia, East Timor, the genocides in Rwanda and Cambodia and the civil wars of West Africa.

The complicated nature of building cases in the absence of international legal precedent and the necessity of gaining support from states for its intervention, as well as the uneven support for the Rome treaty by major powers such as the United States, have undermined the court's efforts to gain acceptance. The ICC and its controversial outgoing chief prosecutor, Luis Moreno Ocampo, have been criticised in some quarters for focusing exclusively on Africa. In an effort to weaken the accusations of an anti-African bias the ICC has appointed Fatou Bensouda from the Gambia to replace the departing Argentinian.

With more than half of the warrants it has issued still unanswered and 11 suspects at large the court badly needed to deliver a concrete return on its annual budget of £90m. The comparative success of more limited tribunals which deal with individual states has led some to believe the ICC is delivering poor value for money.

Defenders of the court, such as Professor Philippe Sands, QC, of University College London, point out that the costs of the ICC are insignificant when weighed against other kinds of spending: "The costs of the Lubanga trial and the ICC as a whole are small compared to the global aid budget."

Court controversy: ICC's high-profile cases

Wanted: The decision to issue the first arrest warrant for a sitting head of state propelled the ICC into the headlines but has failed, so far, to deliver Sudan's Omar al-Bashir to The Hague. The court's critics accuse it of complicating a precarious and complex peace process. President Bashir, top right, has highlighted the ICC's weakness by travelling to countries that helped to create the court without being arrested. Its backers argue that the indictment has been a useful warning.

In custody: Ivory Coast's Laurent Gbagbo, centre right, has not been as fortunate as his Sudanese counterpart. When he refused to concede the presidency after losing an election in 2010, he was eventually forced from power by Ivorian rebel forces supported by France and the UN. He was then handed over to the ICC where he faces war crimes charges over his efforts to stay in power and the conflict they provoked.

The one that got away: After the fall of Colonel Gaddafi the ICC's chief prosecutor was one of the first to board a plane to Libya. However, his pursuit of the Libyan leader's son, Saif al-Islam, left, ended in embarrassment when the authorities told him the captured man would face justice in his home country.