Members of the White Army, a South Sudanese anti-government militia, attend a rally in Nasir, South Sudan, on April 14, 2014. (Zacharias Abubeker/Agence France-Presse via Getty Images)

THE UNITED States played a key role in the arrival of South Sudan as Africa’s 54th nation in 2011. President George W. Bush helped broker a north-south peace agreement in 2005, ending a civil war that claimed nearly 2 million lives. President Obama rallied the world to rescue that peace agreement a few years later when it was falling apart. The creation of the nation was celebrated as proof, as then-Secretary of State Hillary Clinton put it, that “peace is possible if people are willing to make hard choices and stand by them.” Susan E. Rice, the White House national security adviser, said one of her major accomplishments as United Nations ambassador was “helping midwife the birth of the world’s newest nation, South Sudan.”

But now South Sudan is spiraling into the kind of war, chaos and human suffering that Mr. Obama’s appointees — including Ms. Rice and the current U.N. ambassador, Samantha Power — have vowed never to let happen again. Reports are mounting of violence against innocent civilians and mass hunger. The United States, a beacon of hope in the past, now must exercise more leverage and commitment to save South Sudan.

The current crisis began in December 2013 when President Salva Kiir accused his former vice president, Riek Machar, of staging a coup attempt. The conflict mushroomed into a civil war. Some 1.5 million people remain displaced inside the country and 520,000 more have fled across the borders. More than 2.5 million people face food insecurity and the total is rising. War is preventing people from trading, planting crops and moving livestock. The economy is on the verge of collapse. Oxfam, with a team working to provide humanitarian aid, warns that there are “alarming rates of malnutrition and hunger.” Efforts are underway to distribute vital seeds and tools to farmers for the upcoming planting season, but they could be upended by fighting.

The Kiir-Machar conflict always had an underlying tension between South Sudan’s two major ethnic groups, the Dinka and the Nuer. But war has intensified and fragmented ethnic conflict. There are reports of grave human rights violations. An African Union commission of inquiry has reportedly found credible evidence of atrocities. The report, not yet released, should be made public, and no effort spared to find and prosecute the perpetrators.

Peace talks in Addis Ababa, Ethiopia, have failed, with the latest collapse in March. There has been talk of broadening the mediation to include other African states, China, the United States, Britain and others, but so far no real action. Mr. Obama is making a trip to Kenya this summer. Now is the time for the United States to step up to the plate with the same fervor with which it greeted South Sudan’s independence. More than just rhetoric is called for. It is time to pressure Mssrs. Kiir and Machar to lay down their arms; to impose stronger sanctions and an arms embargo; to ensure the delivery of humanitarian aid; and to begin the long, difficult process of healing a young nation midwifed by the United States.