South Sudan owes its existence to the United States.

Years of diplomacy and development aid by successive U.S. administrations helped create the world’s newest nation three years ago. South Sudan was supposed to break the sad, familiar African model of petty rivalries, corruption and oppression. The country has ample oil resources and eager international investors.

But as a conflict fueled by ethnic and personal power struggles tears the country apart, the United States has been unable to pull South Sudan from the brink of civil war.

“We have all of us vowed to do our best to try to prevent that kind of violence. And this is precisely the kind of violence that the people of South Sudan fought so hard for so long to try to escape,” Secretary of State John F. Kerry told reporters before an arm-twisting visit Friday to South Sudan’s capital, Juba. “And the United States and other countries were all deeply involved in the effort to try to make that happen.”

Kerry said South Sudanese President Salva Kiir had pledged to come to peace talks with his rival, former vice president Riek Machar, as soon as this week. Kerry also spoke to Machar by phone Friday.

Although the meeting may never happen, Kiir’s promise to Kerry was the first small sign that Washington has much influence now.

The tensions between Kiir and Machar have been visible for more than a year. Last summer, Kiir fired Machar and the entire cabinet. By December, tensions had boiled over.

Kiir, an ethnic Dinka, accused Machar, an ethnic Nuer, of trying to overthrow him. Machar denied the allegations and launched a rebellion that has split the nation’s army along ethnic lines. The two sides have fought fiercely since then, largely over oil-producing areas, despite a cease-fire signed in January.

A massacre on April 15 by rebel fighters left the town of Bentiu littered with bodies. Soon after, Dinka residents of Bor in Jonglei state attacked a U.N. base where about 5,000 people had sought shelter. At least 58 people were killed and nearly 100 wounded, including two U.N. peacekeepers.

And now, signs are emerging of an impending famine.

In the strongest warning yet from the United States, Kerry said Thursday that the violence is spiraling toward possible genocide.

“Of any nation, the U.S. has the most leverage to bring to bear in South Sudan, but that leverage has been greatly depleted since independence in 2011,” said Cameron Hudson, former director for African affairs at the National Security Council who worked extensively on South Sudan.

Growing tensions overlooked

Some critics say that in its optimism after South Sudan won its independence from Sudan after 22 years of civil war, Washington overlooked the new country’s divisions.

Before its independence, ethnically black South Sudan was defined chiefly by its decades-long conflict with the Arab-dominated Sudanese government in Khartoum. That shared struggle masked some of the divisions among South Sudan’s tribes and factions, although they were never far from the surface.

“The U.S. invested heavily in building institutions of government for the newest country in the world. Meanwhile, though, longtime political divisions within the ruling party were festering,” said John Prendergast, a founder of the anti-genocide Enough Project affiliated with the Center for American Progress.

Prendergast faulted the Obama administration for leaving the job of top envoy for South Sudan vacant for most of last year as security conditions worsened and tensions built. By the time the job was filled in late 2013, “the two competing groups within the ruling party were on a high-speed collision course,” he said.

Current and former U.S. officials acknowledge that the danger signs were there when Kiir and Machar took office, a union U.S. advisers blessed as the best chance to foster unity and national pride. But the speed and ferocity of South Sudan’s descent stunned the Obama administration.

“If it could have been prevented, we would have prevented it,” a senior State Department official traveling with Kerry said of the slide toward civil war. “We saw the problems, but none of us saw that it was going to turn this badly this quickly.”

Some critics say the Obama administration was unwilling to take forceful diplomatic action against South Sudan’s leaders and ignored the warning signs of the looming conflict.

In January, Rep. Edward R. Royce (R-Calif.), chairman of the House Committee on Foreign Affairs, told a congressional hearing that the crisis was “no surprise.” He said experts have been sounding alarms about growing internal tensions since 2011.

But the administration continued to back South Sudan’s leaders.

In 2012, the administration reportedly led an effort to block a U.N. Security Council proposal to impose sanctions on South Sudan for corruption and human rights abuses, Royce said at the hearing.

“Despite these warnings, I am afraid that our investment and diplomatic success may have skewed the judgment of the U.S. government on more than one occasion,” Royce said.

Administration officials place blame squarely on Kiir and Machar.

“A kind of personal violence, a personal anger between two leaders, should never be permitted to take an entire nation in the direction that South Sudan is currently spiraling toward,” Kerry said. “Both President Kiir and Riek Machar need to each of them condemn the brutal attacks that are taking place against innocent people, and they need to condemn the perpetrators of this violence. Leadership is needed.”

Losing patience

Critics said Kerry’s visit to South Sudan was overdue. He has a strong personal relationship with Kiir, having traveled to South Sudan several times when he was in the Senate, and might have used that pull earlier, administration critics said. Kerry spoke to Kiir many times by phone over the past four months but was unable to persuade him to pull back forces.

Now, with few other policy options left, the administration is trying to jump-start an African peacekeeping force that could separate the warring sides, and it is threatening sanctions if the fighting does not stop.

“It is a shame he didn’t engage in this way earlier in the conflict, given the well- understood likelihood that this kind of security crisis could have catastrophic humanitarian consequences,” said Hudson, who is now acting director of the Center for Prevention of Genocide at the U.S. Holocaust Memorial Museum. “I believe it could have helped to avoid a lot of the atrocities we have seen in the past few months had he gone earlier. That said, it is important that he is there now.”

The senior State Department official said sanctions would target the travel and overseas assets of people who could include Kiir and Machar. The U.S. sanctions are already authorized by an executive order signed by President Obama.

But economic and travel sanctions applied only by the United States would have little more than symbolic effect, since few senior South Sudanese of either ethnicity hold large assets in the United States or travel there frequently.

The United Nations also is considering sanctions, and the Obama administration is trying to persuade neighboring nations such as Kenya and Uganda to apply their own. Those could have a significant effect because large banks and institutions are based there.

But despite the efforts of Kenya, Uganda and Ethiopia to foster peace talks, there appears to be little appetite among those nations for economic sanctions.

“They have also lost their patience with both sides,” the State Department official said, but acknowledged that regional sanctions would be a tough sell.

The African Union is beginning a review of atrocities, some of which might eventually be prosecuted as war crimes.

Aside from sanctions, Washington has other options, including the suspension of foreign aid and the downgrading of diplomatic ties.

Kerry didn’t mention those, and U.S. officials hope they won’t need to go that far.

“President Obama’s ‘go-slow’ approach is simply not working,” said Oxfam America President Raymond C. Offenheiser. “The administration had clearly hoped that the threat of sanctions would be enough to encourage the parties to pursue peace talks, but this gambit has failed to stop parties who are fighting for their political — and physical — survival.”