Samantha Power is Barack Obama’s senior foreign policy adviser, but when his office first called her in 2005, she thought, "who is this guy?"

Her 2003 book on genocide, "A Problem from Hell," won a Pulitzer Prize, and she’s a professor at the Kennedy School at Harvard. In a recent interview I asked her why Obama called her. "His office said he had just read my book, and he wanted to talk about, literally, ‘a smart, tough, and humane foreign policy.’ No one from the US government had every called me – no mayor, no school board head."

And why didn’t she know who he was? "I had been out of the country, in Sudan, at the time of Barack Obama’s national coming out, which was the Democratic National Convention of 2004." When she asked around, she heard he was a great speaker. "So I went onto iTunes and downloaded his speech and got on the shuttle down to Washington and listened to the speech on the plane. And I had a cry. I couldn’t believe the speech, couldn’t believe the country he was telling me I lived in.

"And then I went and met with him. We were supposed to meet for an hour. One hour gave way to two, then three. Entering the fourth hour, I heard myself saying, ‘why don’t I quit my job at Harvard and come and intern in your office and answer the phones or do whatever you want?’ It was literally that spontaneous."

Had she been thinking about a job in Washington? "No," she replied. "I had never had any aspiration to go anywhere near government."

What was it about that three-hour conversation that changed her mind? "It was the rigor of the interrogation that I was subjected to," she said. "He really pushed me. Barack is incredibly empirical and non-ideological. He’s very aware of the tectonic plate shifts in the global order – the rise of China, the resurgence of Russia, the loss of influence by the US — and how those affect your ability to get what you want, on anything from global warming to getting out of Iraq to stopping genocide. I thought, if you’re interested in helping change the world in your small way, grandiose as that sounds, even if I was just answering his phones, I would have more impact than writing these big books that I put out ever half decade or so."

I pointed out that a lot of people say Clinton and Obama are pretty much the same on foreign policy. She disagreed. The biggest difference, of course, is "not wanting to go into Iraq in the first place." But beyond that, she said, Obama has "a plan to get out of Iraq responsibly. He is willing to make the Iraqi people central to his plan: to think about moving people from mixed neighborhoods to homogenous neighborhoods if that was required; creating a war crimes commission; giving two billion dollars in aid to Iraq’s neighbors who are sheltering these refugees."

Cuba presents more differences. Obama, she said, "was the first person to come out and say there has got to be a statue of limitations on a failed policy, and surely five decades is enough to know that this isn’t working. So he favors allowing family travel and family remittances as the beginnings of a pathway to normalization.

She pointed to one other difference: "this question of whether we talk to our adversaries without preconditions. Obama said, I’m not afraid of Ahmadinejad. He’s a Holocaust denier, he supports Hamas and Hezbollah, he has infiltrated Iraq, he’s enriching uranium- and by being in the room talking to him, it’s actually being tougher than lobbing these verbal grenades that Bush and Cheney toss from 5,000 miles away. Even if we fail to make progress on any of these issues, we will then have the international wind at our back, and we will have the capacity to mobilize a global response to his regime."

"On all these sacred cows," she concluded, "Obama wants to change the debate, expand the bandwidth" in ways that Clinton does not.

Samantha Power’s latest book is a biography of UN diplomat Sergio Vieiro de Mello called "Chasing the Flame: Sergio Vieira de Mello and the Fight to Save the World."