SOUTH SUDAN, a new nation midwifed into being by the United States out of the ashes of war, is sinking into a mire of senseless violence and chaos. President Salva Kiir and rebel leader Riek Machar are wrecking their country in a conflict that has lasted more than 100 days and threatens more severe human suffering in the months ahead.

Since a schism between the two men triggered fighting between their forces in December, an estimated 1 million people have been driven from their homes, about 800,000 of them internally displaced and 200,000 refugees into Kenya, Uganda, Ethiopia and Sudan. International relief workers are warning that time is running out to avert widespread hunger. South Sudan needs food, water, seeds and farming equipment so that crops can be planted before the rainy season comes at the end of May.

Toby Lanzer, the United Nations aid coordinator for South Sudan, told reporters in Geneva, “If we miss the planting season, there will be a catastrophic decline in food security.” That means hunger, he added, “more grave than anything the continent has seen since the mid-1980s,” during the Ethiopian famine. The World Food Program has begun air-dropping food rations to people stranded between pro-government and anti-government forces. President Obama announced last week that he had signed an executive order that threatens to impose targeted sanctions on those — including South Sudanese officials — who interfere with peace talks, target U.N. peacekeepers or abuse human rights. This is a sad footnote to the soaring hopes that accompanied South Sudan’s 2011 independence.

The cause of all this misery is a bitter conflict between Mr. Kiir, from the majority Dinka ethnic group, and Mr. Machar, of the Nuer group. Their forces have continued fighting despite a January cease-fire agreement. The violence has impeded humanitarian work everywhere in the country. Diseases including measles, malaria, meningitis and diarrheal illnesses threaten the population.

Now, according to reports in the New York Times and Bloomberg News, Mr. Machar’s fighters are mobilizing for a campaign to capture the oil fields in Paloch that are still pumping — the last major source of revenue for Mr. Kiir’s government. A battle for Paloch would thrust South Sudan into still more chaos.

It is not hard to put together a checklist of what should be done. First, Mr. Kiir and Mr. Machar must stand down their forces and participate in a democratic process that includes all. They must stop attacks on humanitarian aid workers and convoys so that desperately needed food and medicine can get through. Rather than a bloody donnybrook over the remaining oil fields, they need to figure out how to restore South Sudan’s potential as a major oil exporter.

South Sudan’s independence was a foreign policy success for the Obama administration, but it is turning into a nightmare. The United States can still help the people of South Sudan build a better future, but first, and urgently, Mr. Kiir and Mr. Machar must step back from the abyss.