As Obama’s longest-serving foreign-policy adviser, Rice, 47, is on a short list of candidates to replace Secretary of State Hillary Rodham Clinton if the president wins a second term. (Stephen Chernin/AFP/Getty Images)

Over Thanksgiving last year, Susan E. Rice, the U.S. ambassador to the United Nations , traveled with her husband and two children to Rwanda. It was part vacation — a chance to see the magnificent bird and gorilla population. But it was also a somber opportunity to visit the memorial honoring more than 800,000 people who died in the largest mass killing in modern times as the world looked on passively.

Rwanda’s 1994 genocide was a watershed for Rice. At the time, she was a 30-year-old National Security Council official in the Clinton administration, which stymied international efforts at the United Nations to respond militarily to the slaughter. Rice took the failure to heart.

But the lessons of Rwanda have offered an imperfect guidebook as Rice weighs the use of U.S. diplomatic and military power. She helped rally support for action in Libya, where the United States helped topple a regime that had threatened to commit large-scale killings. But she has gone the other direction on Syria, with President Obama resisting calls for intervention there as thousands die in the fight against the government of President Bashar al-Assad.

The prospect of international action to halt the violence in Syria will be one of the most pressing issues as the U.N. General Assembly convenes its annual debate Tuesday and Rice moves to center stage as the chief defender of U.S. policy toward Syria as well as the issue of Iran’s ongoing nuclear efforts.

In a lengthy interview last week, Rice said that her image as an interventionist has been overstated and that so has the impact of Rwanda on her deliberations on national-security and foreign-policy issues. Her approach as policymaker, she said, is to “bring analysis and rigor and elbow grease” to every problem.

When Rice arrived in New York in January 2009, she had a reputation as a foreign-policy hawk, a passionate advocate for the use of American military force to halt atrocities in Darfur. While thousands were being slaughtered in the Darfur region of Sudan, she argued for U.S. military intervention in newspaper articles and congressional testimony.

“If anybody thought that I was going to be a bomb thrower or a wild-eyed advocate of military intervention, they don’t know me,” she said. “There is no one-size-fits-all.”

As Obama’s longest-serving foreign-policy adviser, Rice, 47, is seen as a potential candidate to replace Secretary of State Hillary Rodham Clinton if the president wins a second term.

Rice is charming, down to earth, quick with a joke, a proud mother who brags about her children. She was a Rhodes scholar and earned at doctorate in philosophy at Oxford University. As U.S. ambassador, she also has a reputation as a dominant, and sometimes domineering, force among her diplomatic peers.

 She blusters, she cajoles and she curses, participating in high-octave exchanges over Iran and Syria with Russia’s U.N. ambassador, Vitaly Churkin. One council ambassador described her variously as “the bulldozer” and “the headmistress.”

“With Churkin they have this special relationship — they love this kind of dogfight,” said the diplomat, who spoke on the condition of anonymity to avoid antagonizing Rice.

Others have bridled under what one diplomat called her “bossy” personality and frequent absences from Security Council deliberations. During a council trip to Sudan led by Rice, France’s U.N. ambassador, Gerard Araud, groused, “We are not the 14 dwarves, and she is not Snow White.”

White House officials declined to speculate on who would lead the president’s foreign-policy team if he is reelected, but they said Obama believes that Rice has delivered on some of his key priorities, including resolutions imposing sanctions on Iran and North Korea and securing a narrow majority in the Security Council to support the use of force against Libya. They said Russia, not Rice, is responsible for the United Nations’ inability to end the violence in Syria.

“I honestly don’t know who is going to be the next secretary of state,” said Ben Rhodes, the deputy national security adviser for strategic communications. “But the fact is Susan is very aligned with the president and his worldview and has been able to deliver in a very practical sense as U.S. ambassador. I think she is clearly going to continue to be an important adviser to him going forward.”

When the administration needed a surrogate to denounce the YouTube video that sparked violence across the Middle East on the Sunday talk shows a week ago, it turned to Rice. On almost every network, she delivered the administration’s message that the U.S. government had nothing to do with the video. She has come under fire from Republicans for saying the attack in Libya that killed four Americans was not planned, an issue that remains unresolved.

But in New York, critics maintain that Rice’s brash style has sometimes complicated high-wire diplomacy and that her promotion of human rights has been selective.

“She tends to be strongest when the human rights violations involved are committed by U.S. adversaries,” said Kenneth Roth, the executive director of Human Rights Watch. “But she is less strong when violations are committed by U.S. friends, like Rwanda or Israel, or by governments more in the middle, like Sri Lanka.”

Roth said he believes that Rice sought this summer to block the release of a damning U.N. report accusing Rwanda of backing army mutineers in the Democratic Republic of the Congo. He said Rice’s discomfort with the failure of the Clinton administration to stop the genocide in Rwanda created a blind spot in her human rights vision for Rwandan President Paul Kagame, whose military victory ended the slaughter.

Rice was a rising star on President Bill Clinton’s National Security Council in 1994 as the killings were under way in Rwanda and the international community did not intervene. She later told Samantha Powers, a journalist at the time and now an Obama aide, “I swore to myself that if I ever faced such a crisis again, I would come down on the side of dramatic action, going down in flames if that was required.”

Rice told The Washington Post that “it’s not true” that she tried to block the recent Congo report. She said that she merely asked for its release to be delayed to provide Rwanda a fair chance to respond and that she has forcefully criticized Rwanda for its alleged interference in Congo.

A senior administration official said that during her trip to Rwanda in November she delivered the sternest American criticism of Rwandan political repression to date and that she was attacked sharply in the country for doing so.

Her remarks came a day before her family arrived in Rwanda for a vacation and a day after Rice had visited Libya, where NATO-backed rebels ousted Moammar Gaddafi. In her speech, she sought to compare the two countries’ experiences. “Just yesterday, I was in Libya,” she told her Rwandan audience. “I knew from my visit to Rwanda in 1994 that such atrocities were likely in Libya, if Gaddafi went unchecked. I knew we should act, as did President Obama.”

Edward Luck, a U.N. special adviser for Responsibility to Protect, a fledgling U.N. doctrine that obliges states to take action to halt mass atrocities, said he worries that boasting by Washington, Paris and London about toppling Gaddafi has made it harder to strike a deal with Syria’s Assad to protect civilians, setting back the international effort to promote the doctrine. 

“I think they lost sight in both cases of the fundamental issue of the protection of populations over time and started getting into the regime-change thing,” he said.

Rice countered that “there is no way on God’s green earth that had Libya gone down differently the Russians would have been any more ready to allow the Security Council to do anything to put pressure on Assad.”

But with the council unable to act, Rice has pursued a more cautious track on Syria, arguing against U.S. military involvement.

“I’m not of the view that this is a circumstance in which external military intervention is wise for the United States or others,” she said. “I don’t think that [the Responsibility to Protect] was ever a cookie-cutter response that was going to automatically be implemented everywhere things went badly.”