ADDIS ABABA, Ethiopia — For President Obama, the birth of South Sudan four years ago was the capstone of his Africa policy. He sent his United Nations ambassador, Susan E. Rice, for the independence celebration and she took her 13-year-old son to stand in the joyous crowds of the new capital, Juba.
Four years later, that triumph has degenerated into tragedy amid a ruthless ethnic conflagration that has killed tens of thousands of people, displaced more than 2 million others and dashed Mr. Obama’s hopes of forging a brighter future for that corner of Africa. “It breaks my heart to see what South Sudan has become today,” Ms. Rice said earlier this month.
Mr. Obama, accompanied by Ms. Rice, now his national security adviser, will convene a meeting on Monday to try to forge a peace in South Sudan, in his most direct personal intervention since the violence broke out more than 18 months ago. During a visit here to the Ethiopian capital, he will meet with regional leaders to try to build a consensus behind a peace proposal, and to come up with a backup plan in case that fails involving increased sanctions and possibly an arms embargo.
The situation is so grim that White House officials hold out little hope of success. Presidents rarely get involved in a diplomatic meeting without a reasonably guaranteed outcome and Mr. Obama’s aides do not usually talk about a Plan B before a Plan A has failed. But Mr. Obama and his advisers say he had no choice but to try since he was coming to the region anyway to visit Kenya, his father’s home country, and to Ethiopia, where the executive branch of the African Union has its headquarters.
The frustration of Mr. Obama’s staff members, who invested so much in the creation of South Sudan, was palpable as Air Force One arrived here.
“This is a classic case of venal leaders squandering a huge opportunity that they themselves earned, that we all in the international community supported them to obtain,” a senior administration official told reporters on the plane during a briefing with ground rules that required anonymity. “So we can’t undo this for them. They’ve got to fix this.”
The session on South Sudan will follow separate meetings with Ethiopia’s leaders as Mr. Obama becomes the first American president to visit the second-biggest country by population in sub-Saharan Africa. While Ethiopia has been growing quickly, the governing party and its allies have smothered political opposition and the independent news media, arresting journalists and claiming 100 percent of the seats in Parliament in widely criticized elections in May.
Mr. Obama met on Monday with President Mulatu Teshome and Prime Minister Hailemariam Desalegn, and was due to hold a joint news conference later with Mr. Desalegn. On Tuesday, Mr. Obama will address the African Union, the first American president to do so, at a time when he is encouraging Africans to do more to fight the Shabab terrorist group and to resolve other regional conflicts.
The meeting on South Sudan will include Mr. Obama and Mr. Desalegn as well as President Yoweri Museveni of Uganda, President Uhuru Kenyatta of Kenya, Foreign Minister Ibrahim Ghandour of Sudan and Nkosazana Dlamini-Zuma, chairwoman of the African Union. The challenge will be to get regional players that are supporting different sides in the conflict to agree on a unified approach. Uganda, for instance, has been backing the South Sudanese government, while Sudan has been tacitly supporting the rebels.
The conflict in South Sudan, which has become a deadly confrontation between the nation’s largest ethnic groups and tribes, began in a power struggle between two men with a bitter rivalry who had come together to fight a common enemy and form an alliance to forge a new nation. One of them, Salva Kiir, became president of the fledgling country and the other, Riek Machar, was named vice president.
The two come from South Sudan’s main tribes, the Dinka and Nuer, which have fought over land and resources for years. The fragile détente unraveled in December 2013 when Mr. Kiir, a Dinka, accused Mr. Machar, a Nuer, of planning a coup. Their respective security details engaged in a gun battle and within days the nation was consumed by war.
In May 2014, Secretary of State John Kerry traveled to the region to broker a peace deal but the cease-fire he negotiated lasted just days. More than half a dozen other peace plans have fallen apart since then. Mr. Obama’s special envoy, Donald E. Booth, and Ms. Rice have also been involved in trying to broker a resolution.
Fighting has been fiercest in the Upper Nile and Unity States, where the nation’s two major oil fields are found. With the onset of the rainy season, an already dire situation has grown worse.
“Tens of thousands of people are cut off from aid and medical care as fighting intensifies in South Sudan’s Upper Nile State,” Doctors Without Borders, the international humanitarian organization, said in a statement last week.
In Unity State, the devastation resulting from a recent government offensive into the town of Leer, Mr. Machar’s hometown, is just starting to come into focus. Human Rights Watch released a report last week detailing war crimes in chilling detail.
A woman from Koch County described how her two daughters had been raped and then one was tortured. “One man put a gun to the back of my head and said, ‘Watch how we will rape your daughter,’ ” the woman told the Human Rights Watch researcher. She said they had beaten her with a stick, and, after the rapes, had held her older daughter in flames until she caught fire. “She was too injured from her burns so we had to leave her in the bush when we fled,” the woman said, according to the rights group.
The fighting does not appear to be coming to a halt with the rainy season, as it has in the past. On a recent trip to South Sudan, a reporter from The New York Times saw what looked like amphibious assault vehicles being moved out of Juba on trailers. Since then, there have been reports of government forces using “tanklike” vehicles to hunt down and kill rebels and civilians as they have fled into the swamps.
The humanitarian disaster has only worsened. “I am deeply shocked by what I have seen,” Stephen O’Brien, the United Nations under secretary general for humanitarian affairs, said as he concluded a four-day visit recently. The fighting has made South Sudan one of the deadliest conflicts in the world for aid workers, 27 of whom have been killed since the start of the war.
The United Nations’ head of peacekeeping, Hervé Ladsous, urged the Security Council this month to impose an arms embargo “because it is really completely questionable that the very meager resources that the country has go into buying more weapons.”
American officials said they had supported the threat of an arms embargo and might embrace such a measure if the current peace plan was not accepted. “But the thing is we have to find tools that affect the two parties equally,” the senior administration official said, “and the arms embargo is more one-sided than two-sided.”