Irish-born Pulitzer prizewinner Samantha Power is a one-woman think tank on US foreign policy. She tells Belinda McKeonwhy she supports Barack Obama's presidential bid

On the afternoon of August 19th, 2003, in her Boston home, Samantha Power was hard at work on what she hoped would be her second book. Woman for Dark Timeswas the working title; it was set to be a political biography of the philosopher Hannah Arendt.

The plan was to forage in Arendt's life and thought for insights to the situation in which the United States, two years after 9/11 and six months after the invasion of Iraq, found itself gravely mired; Arendt's writing prompted new ways of thinking about the so-called war on terror and about the nature of evil, beyond caricature. It was to be a book, says Power, about "getting into the muck", about head-on, hands-on engagement with the reality of conflict and fear. And sometime that afternoon, as she wrestled with Arendt's writings on totalitarianism and worked to create new perspectives on the political climate which, with every new day of war in Iraq, was becoming ever more troubled, ever more distressing,

Power glanced at the silent television screen in the corner and saw the words crawling along the bottom of the CNN newsfeed: suicide bomb at the Canal Hotel, the headquarters of the UN in Baghdad, dozens dead and injured, Sergio Vieira de Mello trapped under rubble but alive. Power breathed a sigh of relief. It looked like her friend, the UN Commissioner for Human Rights, would live to fight another day.

Having friends in high places is just part of everyday life for Power, who was born in Dublin in 1970 but moved with her family to the US - by way of Kuwait City - at the age of nine. A Yale education, a Harvard law degree, and a Pulitzer Prize later (she won for her first book, A Problem from Hell, in 2003), she is a walking think tank on American foreign policy and human rights, stalked daily for her views by everyone from her students at Harvard's John F Kennedy School of Government to the Washington Post and National Public Radio, to Barack Obama, the Illinois senator who is one of the main contenders for the Democratic nomination in next year's presidential elections. Obama recruited Power to his team in 2005, after he had heard on the grapevine ("through a friend of a friend of a friend", says Power) that she had some ideas on foreign policy that she wanted him to hear; after a dinner conversation in Washington which went beyond its allotted 45-minute slot and into the "wee small hours", Power was convinced to leave her post at Harvard for a year and move to Washington DC to advise Obama full-time.

Enter an Obama who suddenly talked a whole lot more about the Darfur conflict, among other credibility-catching moves; it can hardly be a coincidence that Power was the author of the New Yorker article on Darfur which won the 2005 National Magazine Award for best reporting, or that her Pulitzer-winning book is one of the most thorough and damning dissections of genocide and the governmental machines which feed and sustain it; from Armenia and Cambodia to Rwanda and Srebrenica, A Problem From Hellshows America and its inaction in a stark and unsettling light. Power knows foreign policy, far better, arguably, than those who make and apply it, and, written as it would be in the wake of an event which had not yet happened at the time of her first book - the invasion of Iraq - her second book, whatever its focus, was always bound to comprise another rigorous deployment and a deepening of that knowledge.

And then Sergio Vieira de Mello died under the rubble of the Canal Hotel, and slowly Power came to realise that she had her subject.

"The genocide book [ A Problem from Hell] was used by Cheney and Wolfowitz to justify the invasion of Iraq, to justify American intervention," she says, "which was to use it in a very, very simple-minded way." To see the book read as a plea for "unadulterated military intervention, and for unadulterated US unilateralism", as it has been in some quarters, "horrifies" her, she says. Power warned against the invasion of Iraq before it happened, and she sees Iraq not as an example of the kind of intervention A Problem from Hellurges from American politicians, but rather, as another example of the dangerously casual attitude which led to catastrophe in the past.

TO HER, SHE EXPLAINS, America going to war in Iraq is a "mirror-image" of America doing nothing in the face of genocide in Rwanda. "I mean, both of those are examples where human beings in those countries were singularly irrelevant to questions of what should have been done. Iraq, for me, came not out of a regard for Iraqis gone awry. It was, rather, another reminder, just like Rwanda, of the degree to which we weren't thinking of anybody other than ourselves."

Wilful misreadings were one thing. But the questions which began to come, over the years following the publication of A Problem from Hell, even from readers who fully understood and agreed with Power's stance on America and its responsibilities abroad, suggested to her that there was another lumbering superpower which deserved to be interrogated inside and out. "People are now despairing about the state of mankind and about American foreign policy," she says, "saying, okay, we know America isn't going to do anything about these things; not only do we allow genocide, but we torture people, we do terrible things; but what about the UN? They're saying, my family is saying, why are you so hard on the United States? Shouldn't the UN be doing more?" Answering these questions, Power found, became a more and more difficult process, a more and more demanding scenario; she found herself questioning the very nature of the UN, the way it constantly became "just another diffused source of consolation or disappointment" in the face of desperate need, the ways it was in need of reform, the way it is stuck in the middle of an international system which has, she says, "very little now in the way of adult supervision", very little in the way of leadership and decisive action.

"It just feels like there's chaos afoot in a way that, at least after the Cold War, it felt like even if you didn't like the US, there was someone else out there, and laws would get enforced, and things would sort themselves out," she says. "And now it just feels like it's unravelling."

So just as the history of genocide had served as a lens through which to examine American foreign policy and its failings, the life of Vieira de Mello became a lens for an examination extending far beyond America and looking at crisis and responsibility on an international scale. "Here was a man who was in 10 or 11 major wars, figuring out how to deal with rogues, how to deal with violence, how to deal with justice, what are the viable ingredients for peace or for reconstruction, what is the nature of evil, when do we shun it, when do we engage it . . . " She pauses for breath. "All of these questions that are, lo and behold, actually the central questions of our time. And we need to orient ourselves around answering these questions well, around benefiting from the experience of someone like him. Here's a guy who had a 35-year head start on all the questions that are consuming policy makers today."

HER RESEARCH, SHE SAYS, revealed Vieira de Mello as a "Zelig-like character" who showed up as a broker and a rebuilder in almost every significant conflict from the early 1970s on, from Sudan to the Lebanon, from Bosnia to Kosovo, from Rwanda to Timor to Iraq. "I just had no idea he had so much to offer," she says, "mild-mannered, hot Sergio." She laughs; Vieira de Mello's charisma was described to her, before she met him, as a cross between James Bond and Bobby Kennedy, and as a young reporter outraged by what was happening in Bosnia, she got to see that charisma and its accompanying sharpness first-hand, even in the midst of what was a very bad situation for the UN.

"You couldn't not get on with him," she says. "It's a little bit like what I love about Obama." These are rare people, Power says, adding, with a laugh, that she doesn't think Bertie Ahern is numbered among their ranks. "People who come along and just have such emotional intelligence. Even when they totally disagree with you, they have a way of naming where you're coming from, so that you feel totally seen, completely understood. And that was certainly a great gift, for a conciliator and a uniter." She feels certain that, had Vieira de Mello lived, he would have become secretary general of the UN. She also feels that his death was the day on which the war in Iraq was lost; that day, she says, marked a "turning point" in the war, with the beginning of attacks on civilians (the bombing of the Canal Hotel was the first suicide bomb on a civilian target).

How might things in Iraq have been different had Vieira de Mello lived? "I don't think Iraq would look an iota different," she says. "Not because he wasn't special, but because the catastrophe there was overdetermined. There were so many things done badly from the start that there was no panacea to Iraq by that time. He would have extracted from the situation whatever there was to be extracted, would have talked maybe to Iran, to Syria, but ultimately, the stakeholders had their own agenda. And it's going to take real resources to peel them away one by one."

Power has just finished work on her Vieira de Mello book - it bears the subtitle A Man For Dark Times,so the work she did for her Arendt biography does not seem to have been completely discarded - and she has plenty to consume her while she waits for its publication in February of next year.

For one, there's the run-up to another date next year; the presidential elections in November. Let's hear it straight-up: does Barack Obama have a chance of becoming president? "Here's the good news," Power says, in her best radio pundit's voice. "Barack Obama is going to be president of the United States. Whether it happens in 2008, as I hope it will, or 2012, or 2016 . . . I mean, this is a guy who's got every tool. He's eloquent, he's brilliant, he's handsome, he's empathetic, he's got all the goods. All he doesn't have is 10 years in Washington." That's all? "There are no skeletons with this guy," Power says. "He's clean. And if he loses the election, it won't be because he's black or because his middle name is Hussein."

Neither, Power's determined, will it be because his thinking on foreign policy is anything less than razor-sharp; it needs to be, she says. "Unfortunately, I don't think we have time to spare. I think it has to be now, not least because of the signal it would send abroad - that we have made a radical break with the past."