NAIROBI, Kenya — Rebel fighters seized one of the biggest, most vital cities in the Democratic Republic of Congo on Tuesday, setting off riots in several places across the country and raising serious questions about the stability of Congo as a whole.

The rebel forces took Goma, a commercial hub on Congo’s eastern flank, with little resistance from the national army, which simply fled. Witnesses said United Nations peacekeepers sat in their armored personnel carriers and watched. As the news began to filter across the country, protesters in Kinshasa, the capital, and Kisangani, another major city, poured into the streets, some of them burning buildings, furious that their government was so weak.

In many ways, it was history repeating itself in a country with one of the most haunted, blood-soaked histories in Africa. The trouble goes back more than a century, to when the Belgians waded into this lush expanse in the heart of Africa and brutalized the population in order to extract as much rubber and ivory as possible. In the mid-1990s, rebel forces and several foreign African armies swept through Congo, overthrowing the government and snatching enormous tracts of territory rich in copper, timber, diamonds and gold.

Millions of people died in the ensuing chaos, and back then, just like now, the trouble started in the east.