NAIROBI, Kenya — About one week ago, an enormous column of 8,000 armed youths was advancing through the bush in South Sudan, bent on revenge. United Nations aircraft had been steadily tracking its movements and relaying information back to the head office in Juba, South Sudan’s capital. Several hundred peacekeepers and government soldiers were rushed into place, but the authorities knew they were far outnumbered, and so they told residents to flee.

“This was a massive, overwhelming force,” said Lise Grande, the United Nations humanitarian coordinator in South Sudan.

When the 8,000 rowdy fighters stormed Friday into their destination, the eastern Pibor area, they unleashed a spasm of destruction and violence on a rival ethnic group, burning down huts, looting stores and mercilessly hunting down women and children, witnesses said.

They rampaged for several more days, and the young fighters were even so bold as to trade shots with the South Sudanese Army. But by Wednesday afternoon, the column of fighters seemed to be retreating, heading home with tens of thousands of stolen cows. And though the precise number of deaths is still unknown, a number of bodies have already been discovered, and Ms. Grande said the death toll would probably be “in the tens, if not the hundreds.”