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 bountiful 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 Wednesday, despite the calls for a cease-fire by its patron, Rwanda, the group entered Walikale, a strategic town at the intersection of four provinces in Congo, in the heart of a territory that is home to many gold mines.

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, 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.