SOUTH SUDAN “is on the brink of catastrophe,” a three-member U.N. commission on human rights declared last week after a visit. The chairman, Yasmine Sooka, warned of a repeat of the Rwandan genocide. “There is already a steady process of ethnic cleansing underway in several areas of South Sudan using starvation, gang rape and the burning of villages; everywhere we went across this country we heard villagers saying they are ready to shed blood to get their land back,” she said. “Many told us it’s already reached a point of no return.”

The world responded with a shrug. After long delays, the U.N. Security Council was expected to vote on a resolution Nov. 29 imposing targeted sanctions and an arms embargo, but then Russia, China and others expressed opposition in one form or another, and the vote was put off. An earlier plan to send 4,000 peacekeepers for a regional protection force to join the 12,000 already in South Sudan has yet to be implemented. South Sudan, the world’s youngest nation and an example of U.S. intervention that officials were proud to boast about just a few years ago, is careering once again into chaos. A report from the Council on Foreign Relations says the danger of genocide is real and proposed that the United Nations and African Union run the country for 10 to 15 years to help it rebuild.

Established out of the ashes of a long war in which millions died, South Sudan’s independence in 2011 was a moment of hope but it did not last. The forces of President Salva Kiir and his rival and former vice president Riek Machar went to war with each other in late 2013, a senseless conflict that ended with a peace agreement nearly two years later. But key aspects of the deal have not been implemented, and Mr. Machar fled the country. On July 11, armed men, identified as government forces, went on a rampage in the capital, Juba, at a compound where foreign workers lived, robbing, beating and sexually assaulting them. Mr. Kiir, who has often urged Washington to be patient, appears to no longer be listening to appeals from the United States and elsewhere to stop the violence. The United States only recently backed the arms embargo.

The report from the U.N. team was alarming. Violence is spreading through regions of South Sudan, such as Central Equatoria, that had previously been quiet. Ms. Sooka said she found “an increase in hate speech, a crackdown on the media and civil society, deepening divisions between the country’s 64 tribes,” and a renewed drive for conflict in a nation already flooded with guns and armed groups. She added, “The scale of rape of women and girls perpetrated by all armed groups in South Sudan is utterly unacceptable and is frankly mind boggling.” She said aid workers described gang rape as so prevalent that it has become “normal.”

The U.N. Security Council, the United States and the rest of the world must find a way to confront these atrocities and stop the downward spiral of South Sudan.