UNITED NATIONS — Samantha Power had just boarded a United States Air Force plane on her way home from a three-country tour in Africa when the calls began to come in from Washington. South Sudan, the world’s newest country, had just erupted in civil war along ethnic lines.

That was in December 2013. Ms. Power had taken over as the American ambassador to the United Nations three months earlier, and she wanted to fly immediately to Juba, the capital of South Sudan. What she would do there wasn’t clear, though, and after a series of intense exchanges with Washington, it was decided she would come home.

The back-and-forth was a harbinger of the difficulties Ms. Power would face in trying to avert what the United Nations has said could become a genocide.

Today, the Obama administration’s South Sudan strategy is in tatters. Tens of thousands of people have been killed, the United Nations says, and rape has been rampant.

The starkest diplomatic defeat for the United States came late last month. Ms. Power was unable to persuade the United Nations Security Council to impose an arms embargo on South Sudan and sanctions on key leaders.

“Council members who didn’t support this resolution are taking a big gamble that South Sudan’s leaders will not instigate a catastrophe,” Ms. Power said, citing the world’s failure to respond to genocide in Rwanda in 1994.

But poor timing, bad judgment and a lack of a unified strategy have hampered the administration’s own efforts to avert a catastrophe, many advocates, aid workers and former United States officials say. In turn, it has drawn attention to the limits of American influence — that, too, in a country whose independence from Sudan the United States supported enthusiastically.

It is also a reminder of how challenging it has been for Ms. Power in particular to put into effect the idea that she is best known for: using diplomacy to prevent mass atrocities.

During her three-year tenure, Ms. Power has used her pulpit at the United Nations to denounce human rights abusers, particularly the United States’ rivals. She has used her last days on the job to promote the Obama administration’s diplomatic successes, including dismantling Syria’s chemical weapons and imposing new sanctions on North Korea.

During her three-year tenure as ambassador, Ms. Power has advanced gay rights, pushed the United Nations to do more to stop sexual abuse by peacekeepers and championed the rights of civil society groups. One of her most lauded achievements came early in her term, when she persuaded the rest of the administration to authorize a peacekeeping force to stop mass atrocities in the Central African Republic, a country where the United States had little to gain.

On Syria, she has come under greatest scrutiny because she has had to explain American inaction in the face of atrocities, including the Syrian government’s use of chemical weapons. On Syria, too, she has spoken out most forcefully against Russia. Once, she walked out of the Security Council as the Russian ambassador, Vitaly I. Churkin, was speaking; another time, she accused the Kremlin of “barbarism” for its airstrikes on Aleppo.

She used her last speech as ambassador, on Tuesday, to inveigh against what she called Russia’s “aggressive and destabilizing actions,” from its annexation of Crimea to its meddling in the American presidential election. It served as a warning to President-elect Donald J. Trump without ever naming him. “Russia’s actions are not standing up a new world order,” she said at the Atlantic Council in Washington. “They are tearing down the one that exists.”

For all her impassioned rhetoric, though, Ms. Power has been criticized for using the Security Council more as theater, rather than a platform to resolve conflicts.

In large part, many of her supporters and critics say, her ability to get things done has been limited not just by the widening chasm with Russia but also by her inability to rally the rest of the administration, including on Yemen and South Sudan.

Ms. Power “often made a difference” on issues like the Central African Republic, and gay rights, said Philippe Bolopion, deputy director for global advocacy for Human Rights Watch. But, he added: “Unfortunately, under her watch, atrocities have raged on. While it is unfair to blame any particular individual, the Obama administration was inexplicably gun-shy on sanctions in South Sudan, all too willing to give a pass to Saudi Arabia or Israel for serious violations, and less than creative in overcoming Russia’s obstructionism on Syria.”

In her final words to the Security Council on Wednesday, Ms. Power called the failure to stop mass atrocities in Syria and South Sudan “haunting.”

As for the future, Ms. Power has expressed confidence that South Sudan’s conflict will stay on the Trump administration’s radar. Her would-be successor, Nikki R. Haley, criticized the performance of the United Nations peacekeeping mission during her Senate confirmation hearing on Wednesday, without saying what role the United States should play in stopping the slaughter there.

The outbreak of fighting in South Sudan in December 2013 quickly cleaved along ethnic lines, between forces loyal to President Salva Kiir versus those supporting his former vice president, Riek Machar. Mr. Kiir is a member of the Dinka community, and Mr. Machar is Nuer.

At the time, many advocates knowledgeable about the region pressed the United States to get tougher with South Sudan’s leaders. John Prendergast, a former Africa expert in the Clinton administration who now runs an advocacy group called the Enough Project, said he had recommended targeted sanctions as early as January 2014. By that summer, after United Nations investigators chronicled human rights abuses “on a massive scale,” Human Rights Watch and Amnesty International backed an arms embargo.

Several former American officials said the administration had misjudged the rivalry between Mr. Kiir and Mr. Machar. They also said Susan Rice, the national security adviser, had been reluctant to put too much pressure on them, and in particular to impose an arms embargo. “There are some differences in Washington,” is how Princeton N. Lyman, a former American envoy to South Sudan, put it in March 2015. Some of it, he said later, stemmed from a “lingering sympathy” for the leaders they had empowered.

Asked recently about the differences within the White House, a senior administration official said only that “the U.S. position on this was based on a consensus decision within the interagency process.”

By May 2015, with no peace deal in sight, the African Union asked the Security Council to impose an arms embargo.

A peace deal was signed, only to be broken, repaired and broken again.

In November 2015, the African Union issued a harrowing report that documented massacres, rapes and a sharply deteriorating humanitarian situation.

On the ground, things got worse. By the summer of 2016, after full-scale fighting broke out in Juba, United Nations investigators documented a spate of ethnic killings and said that government troops had been responsible for mass rapes, including of children.

Still, there was no arms embargo and no targeted sanctions against the top leadership. Instead, Ms. Power turned her attention to rallying the Security Council to authorize a surge of 4,000 peacekeepers from neighboring countries to secure Juba.

That effort, said Kate Almquist Knopf, the director of the Africa Center for Strategic Studies, underscored a lack of political strategy. By then, she said, it should have been plain to everyone that Mr. Kiir and Mr. Machar were unlikely to honor any peace deal.

In September, on a trip to Juba, Ms. Power won a promise from South Sudan’s government to allow additional peacekeepers to enter the country.

Their deployment has been delayed by bureaucratic impediments, a lack of visas and conflicting statements by ministers, according to the United Nations chief.

In November, the United Nations’ special adviser for genocide, Adama Dieng, issued the starkest warning: He urged the council to take action to stop what he regarded as steps to genocide.

By then, Ms. Power had won the rest of the Obama administration’s backing to pursue sanctions and an arms embargo. Ms. Rice expressed her support on Twitter, writing, “time is past for an arms embargo & sanctions against those stoking ethnic violence in #SouthSudan.”

But by that time, the African members of the council were not keen to impose sanctions. Russia called the resolution “senseless.” Even some United States allies were reluctant to support it.

Ms. Power issued grave warnings about “the human cost of imposing no cost for attacking civilians.”

Her remarks echoed the dilemma at the heart of her book.

“We have all been bystanders to genocide,” she wrote. “The crucial question is why.”

Source: NYTimes