Samantha Power, President Obama's nominee to be the next United States ambassador to the U.N., spoke bluntly about Syria at her Senate confirmation hearing on Tuesday. Power called the ongoing conflict, which the U.N. estimates has killed more than 90,000 people and produced 1.8 million refugees, "one of the most devastating cases of mass atrocity" she has ever seen.

"I don't know that I can recall a leader who has, in a way, written a new playbook for brutality in terms of the range of grotesque tactics that the Assad regime has employed in response to a democratic uprising," she told members of the Senate Foreign Relations Committee.

Power wrote the Pulitzer Prize-winning book, "A Problem from Hell," a comprehensive look at the role the United States and the United Nations has played in the world's biggest genocides. Her journalism and international activism have included travels to Bosnia and Sudan. In her testimony, she pointed to what has largely been seen as inaction by the U.N. Security Council on Syria as a symbol of the organization's dysfunction.

"We see the failure of the U.N. Security Council to respond to the slaughter in Syria - a disgrace that history will judge harshly," she said.

Most recently, Power served as part of President Obama's National Security Council staff, working as a chief adviser on human rights issues. During the hearing, she stood by the administration's continued assessment that President Assad will eventually fall.

"History shows that regimes that brutalize their own people in that manner, that totally force it, their legitimacy, that do not abide by even basic norms of human decency, they just do not have the support to sustain themselves," she told the committee. "So the day of reckoning will come."

Power seemed prepared for questions over previous statements she's made that were critical of Israel, assuring the senators that she "will stand up for Israel and work tirelessly to defend it."

But she also faced some criticism from Republican senators over her past perspectives on American foreign policy, in general. She appeared at times to struggle in reconciling her activist past with her most recent roles working for and within the very institutions she spent so many years being critical of.

In particular, she was questioned about a 2003 New Republic article in which she wrote that American foreign policy needed "a historical reckoning with crimes committed, sponsored or permitted by the United States."

When Sen. Marco Rubio. R-Fla., asked Power several times to name exactly which crimes she was referring to, she repeated, "America is the greatest country in the world and we have nothing to apologize for."

After more questioning from Sen. Ron Johnson, R-Wis., about her 2003 article, Powers said that her perspective has evolved over the last decade.

"I have written probably 2 million words in my career. There are things I have written that I would write very differently today," she said. "Serving in the executive branch is very different than sounding off from my academic perch."

Even with the criticism, she was expected to be easily confirmed. At the hearing, both Democrats and Republicans praised Power's human rights background and outspokenness.

"We have lots of people who come before us, some of which are more interesting than others," said ranking member Sen. Bob Corker, R-Tenn. "I have a feeling that you certainly are going to carve a path at the United Nations. I look forward to watching that."