"To those who care deeply about America's engagement and indispensable leadership in the world, you will find no stronger advocate for that cause than Samantha," Obama told reporters in announcing the nomination.
Rice has been pilloried by Republicans in Congress during her time as U.N. ambassador for initially saying that the September 2012 attacks against a U.S. diplomatic facility in Benghazi, Libya, likely were spontaneous, which turned out to be wrong. Four Americans, including U.S. Ambassador Chris Stevens, were killed in the attack.
Power, a native Dubliner who grew up in Pittsburgh and Atlanta, likely will have an even harder time watching her words. Reflecting on her time in the early 1990s as a war correspondent in Bosnia, Power said she returned to the U.S. "very sort of dispirited about the power of the pen, very dispirited about the United States and foreign policy."
"The battle to stop genocide is lost in the realm of domestic politics," Power said in the same June 2002 interview with C-Span, to discuss her Pulitzer Prize-winning book, "A Problem From Hell: America and the Age of Genocide." "And that is where it can be won if it's to be won."
Her self-written book summary was even more blunt: "The United States has never intervened to stop genocide," she wrote.
Most governments have been careful not to label the civil war in Syria, which has killed more than 70,000 people and is not now in its third year, as genocide. Qatar and Turkey, however, have accused Syrian President Bashar Assad of committing war crimes against his people. And rebel fighters who are pleading in vain with the White House for weapons and other military aid have accused the regime of launching a "massive genocide" across the country.
The White House has shown little interest in supplying the rebels with arms or using U.S. force directly against Assad. How Power will balance her past of pushing for U.S. intervention against Obama's current recalcitrance on Syria and not get caught in a domestic political dispute over it will be a delicate line for her to walk at the United Nations.
Within minutes of her nomination, Sen. John McCain, R-Ariz., predicted Power will win an easy Senate confirmation, calling her "well-qualified for this important position." McCain is one of the leading proponents of U.S. intervention in Syria.
"The hope is that her experience will give her the expertise and ability to address situations of mass atrocities in Syria effectively, and to be persuasive within the administration," said Peggy Hicks, the global advocacy director at Human Rights Watch and a longtime Power friend and colleague.
Hicks added: "That doesn't mean intervention, necessarily. It could be a broad range of strategies that could be effective in addressing human rights crises, like in Syria."
As Power tells it, she was a 23-year-old war correspondent for a variety of publications in the Balkans when she realized that the Clinton administration was refusing to stem ethnic cleansing and genocide in Bosnia and Rwanda. She described Rice at the time as a "rising star" on the National Security Council who allegedly balked at describing the atrocities in Rwanda as genocide for fear that inaction there would hurt Democrats in the 1994 congressional elections.
Rice has said she does not recall making the comment, and the two women appear to be close now.
Power returned to the U.S. to pursue a law degree at Harvard University, where she remained as executive director of a human rights policy center and wrote her prize-winning second book. She began working for Obama in 2005 as a foreign policy adviser, and also served on his first presidential campaign, when Power described Clinton as "a monster" for, she said, fear mongering to voters in Ohio. Power resigned from Obama's campaign, and apologized to Clinton shortly after his victory.
She remained influential in the White House as a senior national security staff adviser to Obama, overseeing human rights and multilateral affairs, and stepped down only earlier this year to care for her two young children.
James Mann, who studied Obama's first-term national security team as author of "The Obamas" and is now at the John Hopkins' School of Advanced and International Studies, said Power is "very much in line with Obama's own mentality and way of thinking."
Noting that Obama appears uninterested in intervening in Syria, Mann predicted that Power will have to follow suit.
"The framework that she would bring to this is that there has to be other ways of dealing with Syria in additional to military intervention as alternatives if the president doesn't want to take military action," Mann said.
Power made clear Wednesday, however, that she will remain a watchdog at heart in her new post, referencing both U.N. successes in Sudan and failures in Bosnia.
"The question of what the United Nations can accomplish for the world and for the United States remains a pressing one," she said. "As the most powerful and inspiring country on this Earth, we have a critical role to play in insisting that the institution meet the necessities of our time. It can do so only with American leadership."