More than three million people died in the Democratic Republic of Congo's five-year “civil” war. Eight other African nations were dragged into a miasma of horrifying violence. In 2003 they agreed to a ceasefire, but the bloodshed, famine and disease continued: another 1.4 million have perished since. Now the region is at risk of stumbling into full-scale conflict once again. If it does, the UN's threadbare reputation as a peacekeeper will be the least of the casualties.

The immediate crisis is focused on Goma, a city of 600,000 where paradise can seem to cohabit with hell on Earth. Nearby, forested mountains fringe the smallest of Africa's Great Lakes. Yet Goma is also the place where Rwandan Hutus dropped their machetes while fleeing after a genocide that left nearly a million Tutsis dead.

Yesterday a force of Congolese rebels led by General Laurent Nkunda, an ethnic Tutsi, was advancing on Goma with the stated aim of “liberating” the entire country. Government troops have abandoned the city. Refugees, who have more than doubled in number to at least 45,000 in the past two days, choked the area's few roads. The UN's 17,000-strong peacekeeping force, its biggest anywhere, was “stretched to the limit” with no peace to keep.

Ban Ki Moon, the UN Secretary-General, has demanded an end to the fighting. He and the EU have dispatched envoys to Kinshasa. France has suggested sending something more muscular but entirely untested - an EU battlegroup.

In fact there are no easy answers to the questions that Congo poses. Three years ago Jan Egeland, then the UN's emergency relief co-ordinator, called the country's continuing violence the world's worst humanitarian disaster. There are three main reasons why it still is. First, it owes its origins directly to the Rwandan genocide. The mass murder of General Nkunda's fellow Tutsis by Hutu militias allows him to disguise what is largely warlordism as the defence of a minority, just as it has provided the rationale for two Rwandan invasions of Congo since 1994.

Secondly, the world's resolve to prevent a repeat of the genocide gave way swiftly to a masterclass in neglect. The UN has brought some génocidaires to justice, but its force in Congo has been mired from the outset in sexual abuse scandals and the speed of the country's descent to the status of failed state has been matched only by the depth of diplomatic indifference.

Thirdly, Congo's immense natural wealth, especially in precious metals, has given its leaders, neighbours and investors powerful incentives to manoeuvre for control of territory but none at all to worry about its people.

Yet the very same factors offer a glimmer of hope. President Kagame of Rwanda has some moral sway as an avenger of the 1994 genocide that he should use to try to restrain General Nkunda. The UN must heed its own officials' pleas for more and better peacekeepers. And China, the biggest investor in Congo's minerals, must accept a concomitant responsibility to lead efforts to prevent a slide back to war.

Congo remains Africa's dark heart, and an object lesson in failed decolonisation. But it also represents an opportunity for emerging powers who chafe at what they see as Western dominance of the UN. Where so many have failed, they have a chance to lead.