Rare-earth minerals critical for smartphone manufacturing. Lucrative trafficking routes and dizzying stockpiles of weapons. The lives of millions of people. All are now under the control of the M23 militia and its powerful backer, Rwanda.

M23 reigns over a vast territory in eastern region of the Democratic Republic of Congo, home to lucrative mines and other natural resources. In the major city of Goma, on the border with Rwanda, M23’s soldiers now patrol the streets and M23-appointed officials rule the city. Congo’s large but inept army has not slowed the group’s advance, nor has condemnation by the United Nations Security Council.

After months of fighting, the leaders of Congo and Rwanda held talks in Qatar this week and called for an immediate cease-fire. M23 declined to comment on whether it would honor the cease-fire.

Last month, The Times traveled to Goma days after its capture by M23.

M23, once a ragtag militia, now behaves like a governing entity in buzzing cities, lucrative coltan and gold mines, and strategic border crossings. Its immigration officers stamp passports, and in a city still scarred by deadly fighting, its leaders have urged young people to join its army so they can “liberate Congo.”

The group has vowed to march on Kinshasa, Congo’s capital. That makes M23 and Rwanda a threat to the sovereignty of Congo, the biggest country in sub-Saharan Africa by territory, with more than 100 million people, where millions of people have died in the last three decades in endless wars.

A powerful armed group

M23 — named after the March 23, 2009 date when it claims the Congolese government failed to honor an earlier peace agreement — has 6,000 to 9,000 fighters, according to the United Nations. Experts say the group is growing more powerful and sophisticated.

“I’ve dealt with the Houthis in Yemen and rebel groups in the Central African Republic, but this tops everything I’ve seen,” Vivian van de Perre, the deputy head of the U.N. peacekeeping force based in Goma, said about M23’s military capabilities and political ambitions.

M23’s political leader is Corneille Nangaa, the former head of Congo’s electoral commission. He claims the group can provide justice and safety to a long-suffering population.

“Congo’s problems come from a lack of state’s authority,” Mr. Nangaa said in a recent interview with The New York Times Times in Goma. “Our goal is to rebuild the state.”

A burly man in his mid-50s, Mr. Nangaa was sitting in the lush garden of the former governor’s house on the shores of Lake Kivu, a Congolese flag behind him. Days earlier, bodies of people killed by his fighters had washed up on the lakeshore.

In areas it has seized over the past year, M23 has carried out forced recruitment, including of children, extrajudicial killings, and sexual violence, according to U.N. researchers.

Mr. Nangaa has claimed he will “get rid of corruption and disorder.” According to the United States, which has twice sanctioned him, he oversaw the embezzlement of more than $100 million when he ran Congo’s election commission and oversaw the election of President, Felix Tshisekedi.

In 2023, Mr. Nangaa fell out with the Tshisekedi government and created a coalition of rebel groups dedicated to overthrowing his former allies. That includes M23, the coalition’s military arm in eastern Congo.

In early February, he promised tens of thousands of people gathered in a Goma stadium that justice, safety and development would soon prevail.

“We’re freed, we have security,” said Serge Abeli, 25 and unemployed, as he left the stadium. “Now the new leaders can tackle unemployment and the cost of living.”

But Mr. Nangaa didn’t mention the one thing Congo’s people yearn for the most.

“We just want peace,” said Célestin Selemani, 29 and also unemployed, on a Goma street on a recent morning. “I grew up with war and I’m tired of it.”

Minerals bound for Rwanda

The conflict has its roots in Rwanda’s 1994 genocide, which spilled over the border into Congo.

Like the leaders of Rwanda, M23 is mostly made up of people from the Tutsi ethnic group, who were targeted in the genocide. It has claimed it is in eastern Congo to protect fellow Tutsis from persecution. But according to a United Nations report, M23 is actually planning for “territorial expansion and the long-term occupation and exploitation of conquered territories.”

And since the beginning of the year, some armed groups mainly made up of ethnic Hutus have joined M23, according to experts, showing the group’s appeal beyond Tutsis.

The Rubaya mine, northeast of Goma, has one of the world’s largest deposits of coltan — a mineral containing the “rare earth” element tantalum, used in smartphones, knee replacements and explosive devices. Seen from the sky, the mine looks like a giant shovel has scarred the earth with thousands of tiny holes and streaks to extract its flesh.

M23 collects at least $800,000 monthly in taxes from coltan production, according to U.N. experts, who estimated that at least 150 tons of the mineral have been fraudulently exported to Rwanda.

(Mr. Nangaa called the $800,000 estimate “a joke.”)

The United Nations calls M23 a proxy army for Rwanda, with units directed by Rwandan soldiers and equipped with weapons supplied by Rwanda’s military — antitank missiles, sophisticated automatic rifles and spoofing devices, among others.

President Paul Kagame of Rwanda denies backing the group, but the thousands of Rwandan troops present in Goma and across eastern Congo leave little doubt, according to a dozen intelligence analysts, diplomats, researchers and humanitarian workers who interact with the group or study it.

Mr. Nangaa condoned the presence of Rwandan troops in eastern Congo during his interview with The Times, arguing that Congo’s problems threatened other countries.

“If we are neighbors and your children throw stones at me over the fence, I’ll tell you to ask your children to stop throwing stones at me,” he said. “If you don’t manage them, I’ll deal with it myself.”

M23’s iron-fisted rule

It remains unclear if Mr. Nangaa will actually march on Kinshasa. Analysts say that a more realistic target might be the effective control of eastern Congo, an area roughly the size of Greece or Louisiana.

“The more territories they take, the more bargaining power they have,” said Stephanie Wolters, a specialist on Congo at the South African Institute of International Affairs.

M23 has imposed strict public order in newly conquered territories, but its control has remained fragile. Last month, an attack at an M23 rally attended by Mr. Nangaa in the city of Bukavu killed at least 11 people and injured dozens more. No group claimed responsibility for the attack.

M23 has also used brutal methods, according to residents, civil society activists and human rights defenders.

An artist was shot dead on the street as he was recording a video clip for a song in which he accused M23 of invasion, looting and sexual violence, according to an eyewitness and an activist who knew him. M23 has denied involvement.

M23 has also refused to reopen Goma’s airport, depriving aid groups of their main lifeline to a city that until recently hosted more than a million displaced people.

Instead, shortly after Goma’s capture, M23 ordered displaced people sheltering in and around the city to return to their homes, arguing that the territories under M23’s control were now safe. Humanitarian workers have warned that this could create another wave of displacement.

Pascaline Furaha, a 25-year-old mother of four living in a camp on Goma’s outskirts and expecting her fifth child, said she couldn’t return home near Rubaya because of recent fighting there.

Despite the unstable security situation, M23 expelled nearly 700,000 people from the camps, according to U.N. figures. Tens of thousands of white tents dotting Goma’s hills, each one sheltering a family like Ms. Furaha’s, were dismantled in just a few days.