What is happening in eastern Congo, where rebels claim they captured a key city?

Rwanda-backed rebels have claimed control of eastern Congo’s strategic city of Goma, the hub of a region containing trillions of dollars in mineral wealth that remains largely untapped.

It marks a sharp escalation in one of Africa’s longest wars, threatening to dramatically worsen a dire humanitarian crisis.

On Tuesday, U.N. officials reported bodies lying in the streets, widespread looting and outages of electricity, water and…