The fighting, which began in April, is the worst in the DRC for five years.
It has effectively wrecked efforts to negotiate an end to a war that has raged more or less incessantly for 18 years, largely over the DRC's vast mineral wealth.
The current bloodletting centered on Kivu province is in large part a consequence of the 1994 genocide in neighboring Rwanda in which majority Hutus massacre some 800,000 minority Tutsis.
The government-orchestrated killing spree, carried out over 100 days, was the culmination of a long-standing animosity between the two ethnic groups.
Many Rwandans, including hundreds of thousands of Hutus, fled into the DRC, then known as Zaire.
Since then Congo has been engulfed by war, in which several million people have been died, many from famine and disease. Atrocities abound and tens of thousands have been slain in the savage tribal bloodbath.
Rwanda has played a major part in this war, the bloodiest conflict in Africa for decades.
Invasions by Rwandan forces, and Rwanda's backing for Congolese rebels, fueled the conflict that over the years has dragged in a dozen African states, all plundering Congo's mineral riches through state forces or militia surrogates.
Congolese President Joseph Kabila and his Rwandan counterpart, Paul Kagame, engineered a peace plan in 2009. But that fell apart when M23 rose up against the Kabila government in Kinshasa, 900 miles west of Kivu, in April, when Kabila came under international pressure to arrest one of its leaders, a renegade general called Bosco Ntaganda, aka "the Terminator.
He's wanted by the International Criminal Court on war crimes charges.
"His faction had served as the protector of economic interests for powerful members of ethnic Tutsi elites on both sides of the Rwandan border, and as a line of defense against rival ethnic militias in the region," the Financial Times observed.
Human Rights Watch and the United Nations alleged in June that Rwanda was backing Ntaganda, providing his forces with troops, arms and ammunition, a charge Kagame's government denied.
But Western powers, including the United States, Britain and the Netherlands, which have long supported Kagame, cut off aid to his post-genocide regime in Kigali, Rwanda's capital, in July.
The West's guilt at its failure to stop the 1994 genocide was the primary reason for its unstinting support for Kagame over the years.
Rwanda, with its flourishing economy built on development aid, became the darling of the donor community and seemingly could do no wrong.
But all that has evaporated now, as the greed and lust for power among Africa's dictatorial leaders has become unavoidably evident.
Kagame won international applause for using development to reduce poverty in Rwanda and foster economic growth. But his reputation was further tarnished June 21.
Rwanda's former army chief, Gen. Faustin Kayumba Nyamwasa, testified during a court trial in South Africa that Kagame had ordered the assassination of Juvenal Habyarimana, Rwanda's president in 1994, which triggered the genocide.
Kagame's regime has denied involvement in Habyarimana's death, which occurred when his aircraft was shot down by rocket fire over Rwanda.
But an earlier French investigation of that mystery-shrouded incident concluded that Kagame ordered the attack on Habyarimana's plane and that the missiles were fired from an area controlled by Kagame's forces.
On Sept. 11, another Human Rights Watch report accused M23 rebels of committing war crimes, including summary executions, mass rape and the force recruitment of child soldiers in the region they control north of Goma, where the U.N. peacekeeping force has its headquarters.
It claims that two-thirds of some 900 new fighters, some under the age of 15, that recently reinforced M23 were conscripted in Rwanda, many of them by force.
Human Rights Watch said that Rwanda's alleged involvement with M23 makes the Kigali regime itself liable to war crimes charges.
It says the U.N. Security Council should consider imposing sanctions on senior Rwandan officials.
"From a legal perspective, because Rwanda is directly involved in the conflict both in providing recruits and fighting it could be made accountable for war crimes," said Anneke van Woudenberg, Human Rights Watch's senior Africa researcher.