The militia, made up largely of Congolese army mutineers and veteran jungle fighters, took Goma Tuesday and declared they were ready to "liberate" the vast Central African state, which has been wracked by war since 1996, and march on Kinshasa, the capital.
"We're now going to Kinshasa," declared M23 Col. Vianney Kazarama in Goma. "No one will divide this country."
M23 is also targeting the major city of Bukavu, southwest of Goma. If rebels capture it, they would effectively control the entire eastern region.
There were heavy diplomatic efforts at the United Nations and among Central African nations to head off a greater conflict in the region.
As it is, the fall of Goma and its airport was a major blow to United Nations-led efforts to halt the fighting that began with an April rebellion by M23 against the government of President Joseph Kabila.
The seizure of Goma, a major defeat for Kabila, "could relaunch open warfare between the DRC and Rwanda," the International Crisis Group warned Tuesday.
On Wednesday, the U.N. Security Council unanimously condemned the seizure of Goma, a city of 1 million on the western shore of Lake Kivu. The council asked U.N. Secretary-General Ban Ki-moon to report on what external support the rebels have received.
Kabila's regime in Kinshasa claims Rwanda masterminded the April rebellion by disaffected troops as a way to get its hands on the eastern DRC's vast mineral wealth, as so many have done since the 19th-century colonial powers began plundering the region.
The DRC, formerly known as Zaire, has large deposits of diamonds, gold, coltan (which is used in mobile phones) and others minerals such as tin and tungsten ore. These riches are financing multiple armed groups, which garner hundreds of millions of dollars per year trading in stolen minerals.
A leaked U.N. report alleges Uganda is also aiding the rebels. The Kampala government denies that and has threatened to withdraw its forces in Somalia aiding the U.N.-backed transitional government against Islamist insurgents.
If those 5,000 troops, one-third of an African Union peacekeeping force, are pulled out only weeks after they hammered the Islamists of al-Shabaab, the shaky Mogadishu government would be dangerously vulnerable.
The United Nations itself was roundly denounced by France, which once ruled a vast colonial empire in Africa.
U.N. peacekeepers in Goma made no effort, after some initial helicopter gunship attacks, to engage the advancing rebels after Congolese government forces abandoned the city.
Thousands of Kabila's troops and police officers reportedly defected.
The U.N. troops, which remain in Goma, are part of 17,000-person force known as Monusco.
The United Nations backs Kinshasa, as do neighboring powers like Angola, the Republic of Congo and Tanzania.
Kinshasa has accused neighboring Rwanda, whose army has repeatedly intervened in the Congo's conflicts over the last 15 years, of backing the rebels with troops, arms and supplies.
Rwanda denies it's aiding M23 but in recent years it's invaded its mineral-rich neighbor three times and claims the DRC fires into its territory.
"M23 is a Rwandan proxy; it is just as likely that Rwanda will help M23 hold Goma and even extent to Bukavu," observed Oxford Analytica.
"This would signal effective Rwandan annexation of the Kivu region, consistent with its desire for a western border buffer zone, and mineral extraction interests."
On Wednesday, the Security Council approved a French resolution calling for an immediate M23 withdrawal and the cessation of outside help for M23. Neither is expected to happen.
War erupted in the DRC in November 1996, triggered by the genocide in Rwanda in which Hutu tribesmen slaughtered some 800,000 Tutsi rivals.
That war ended in May 1997, but flared anew in August 1998 into what's known as the Second Congo War.
It's also known as the Great War of Africa because it directly involved at least eight African nations and around 25 armed groups. About 5.4 million people have died, largely from disease and starvation, making it the deadliest conflict since World War II.