Uganda and Rwanda said they entered the civil war in the Democratic Republic of Congo in support of the rebels to prevent a genocide, but in the Ugandan-controlled north-east aid agencies are reporting massacres on a chilling scale.
UN agencies estimate that between 5,000 and 7,000 people have been killed since June in the countryside around the town of Bunia, while another 150,000 are thought to have been displaced by the fighting. One aid agency has warned that genocide looms and has released a video of burning villages and dead or dying women and children with horrific machete wounds.
The conflict - between the cattle-herding Hema and the farming Lendu tribes - has parallels with that between minority Tutsis and majority Hutus in Rwanda, which exploded into genocide in 1994. There have long been tensions between the Lendu, who are more numerous in the area but much poorer, and the richer Hema.
The root of the conflict is competition over land and the food, coffee and gold it yields. The first major outbreak of fighting came in June with a Lendu attack on Hemas, who, they said, had stolen their land.
But it is also a conflict about political power in Laurent Kabila's new, divided Congo, and a story of how a foreign army upset a delicate tribal balance.
Congo's natural wealth has always been its curse, from the time Belgium's King Leopold created and then raped the country in the 19th century in a frantic quest for rubber and ivory. Today, civil war in the former Zaire has sucked in foreign armies from Zimbabwe to Rwanda - and some seem to be there just for a share of the spoils.
Their divide-and-rule tactics, or plain insensitivity to local rivalries, have often pitted Congolese against Congolese. When the Ugandans appointed a Hema woman last year as governor of the newly created Kibali-Ituri province, many Lendu felt hard done by.
"There were some Hemas who decided to try and translate their economic and financial power into a monopoly of political power and then exclude the Lendu," said a leading rebel official in Bunia.
Amnesty International says Ugandan soldiers took money to fight for the Hema, and others sold them guns. But most of the blame, observers say, probably lies with local leaders playing tribal politics for their own ends.
"In all these conflicts in the region, you have a small elite which needs support, and the easiest way to get support is to call on ethnic sentiments," Amnesty's Godfrey Byaruhanga said.
Both sides are guilty of massacres since the conflict began, with the poorer Lendu tending to use traditional weapons such as machetes, arrows and spears, and the Hema guns. "What exacerbates the conflict is the fact the violence is taking place in a quasi-political vacuum," Byaruhanga said.
The Hema-Lendu conflict is only one of the many ethnic problems which the Congolese civil war has reignited.
The rebel movement, for example, is itself seen as dominated by Tutsis from the east or abroad. When rebels tried to take the capital in 1997, innocent Tutsis were lynched in the streets of Kinshasa on the basis of their ethnic identity alone.
Although a peace deal was signed in Zambia by the numerous belligerents last year, fighting in Congo's interior continues and the country is still waiting for a promised UN peacekeeping force.
The Security Council is working on a resolution to authorise a UN force, but memories of a disastrous intervention in Somalia in 1993 could still delay its deployment.