ONE morning in October 2009, Barack Obama woke up to an unusual problem: he had won the Nobel Peace Prize.
The international community's highest award for compassion would normally be a welcome boon. But for opponents of the fledgling Democrat president the prize fit into a neat caricature.
Obama was a European-style peacenik, they argued, feted in the salons of Scandinavia for his poetic speeches but too thin-framed to bear the burdens that come with leading the world's military superpower.
In the weeks that followed, Mr Obama wrestled with what to say when he accepted the prize in Oslo and how to at once address the internationalists in the hall and the sceptical American audience at home.
As the drafts of the speech piled up, he turned to Irishwoman Samantha Power, a former war correspondent who left journalism to advocate for human rights and genocide prevention before joining the White House staff, to help him make the case that war is sometimes the answer.
Nearly four years later, the version of just war theory that Mr Obama laid out is being tested to the point of destruction by the killing in Syria, where an estimated 100,000 have already died.
The President is turning to the 42-year-old Ms Power, this time as his nominee as US ambassador to the United Nations.
Ms Power was born in Castleknock, Dublin to doctor parents who separated when she was a child. Her father, Jim Power, died before she emigrated to Pittsburgh aged nine with her mother, stepfather and brother.
Ms Power claims she arrived in the US wearing a stars-and-stripes t-shirt. Her story is an accelerated version of the successful immigrant tale: straining to lose her Irish accent, winning the acceptance of her childhood peers, excelling at sports and then earning a place at Yale.
After graduating, she briefly bounced around Washington as an intern but by 24 was in the remains of Yugoslavia as a freelance reporter, covering the atrocities being committed by Serbian forces in Bosnia.
Ms Power stood out in a mainly male journalistic pack, many of whom were experienced war correspondents. But she earned their respect for her dogged reporting and dark sense of humour.
John Sweeney, a BBC correspondent who covered Bosnia, said that while some journalists focused on the tragedies in front of them, Ms Power was probing the international angle and asking what it would take to make the world intervene to stop the killings.
"She was smart and she had a real insight into part of the story that other reporters were in the dark about: the question of where does America stand on this?" he recalled.
That question - how to convince the US to intervene in foreign horrors when its immediate interests were not at stake - would occupy Ms Power and form the basis for her Pulitzer Prize-winning book "A Problem from Hell: America and the Age of Genocide".
Even before leaving Bosnia, she was moving from a reporter to an advocate, who had no reservations about haranguing US officials about the need to stop what she recognised was widespread ethnic cleansing.
Despite her pestering questions about US non-intervention, many of the American foreign policy operatives she encountered took it on themselves to help guide her promising career.
She studied at Harvard Law School and then taught at the university as she began to publish books. In 2005 her connections put her in touch with Barack Obama, then a promising freshman senator.
To the surprise of many who knew her, the fiercely-independent Ms Power agreed to give up her public profile for a relatively low-level job in his senate office.
"It was a little surprising to me how worshipful she was of Obama," said Peter Galbraith, who knew her when he was US ambassador to Croatia. "She's deeply devoted to him in a reverential sort of way that probably Samantha the journalist probably would never have been."
When Mr Obama declared his White House ambitions, Ms Power followed him from Washington onto the campaign trail.
As well as advising on foreign policy, she joined a growing band of volunteers knocking on doors for him in Iowa, the Midwestern state where Mr Obama would go on to defeat Hillary Clinton in the first battle for the Democratic nomination.
It was there that she met and began dating Cass Sunstein, one of America's foremost legal scholars and a former colleague of Mr Obama's from the University of Chicago Law School. Like her, Mr Sunstein had thrown scepticism aside and thrown his lot in with the first-term senator from Illinois.
The pair were married in Ireland in July 2008. Mr Sunstein later became the President's regulatory czar, prompting Glenn Beck, the conservative radio host, to describe the pair of liberals as "the most dangerous couple in America".
By that stage, Ms Power was no longer part of the Obama operation. Weeks earlier she had blurted out in front of a reporter that Mrs Clinton was "a monster" before hastily trying to claim the comment was off the record.
It was an unusual mistake for a former journalist and despite their personal connection, the senator did not hesitate to accept her immediate resignation from the campaign.
By the time Mr Obama entered the White House in January 2009 she had been sufficiently rehabilitated that a staff job was waiting for her in his National Security Council.
For the first time, Ms Power was in a position within government to put into practice the interventionist policies she had so long advocated for. The chance soon came.
By March 2011 the uprising against Muammar Gaddafi had spilled over into full civil war and regime forces were massing to storm Benghazi, the rebel's stronghold in eastern Libya.
Ms Power was among what some commentators dubbed "Obama's Valkyries", a group of female foreign policy leaders urging the president to step in.
Other members included Mrs Clinton and Susan Rice, the woman Ms Power will replace as US envoy to the UN, subject to confirmation by the senate, as they faced up against more sceptical male advisors.
Their argument prevailed and the US joined with European allies in launching airstrikes against Gaddafi's troops. One White House aide recalled seeing Ms Power in the hallways looking drawn and tired as the bombing campaign commenced. "Overthrowing dictators is stressful," they joked.
The Libyan intervention succeeded in removing Gaddafi, and satisfyied both the humanitarian desire to prevent a slaughter in Benghazi while also keeping American troops from getting entangled in another Middle Eastern ground war.
But if confirmed as UN ambassador, Ms Power will be at the forefront of dealing with America's response to the ongoing conflict in Syria - this time facing a far more complex situation on the ground and a president adamant about keeping the US out of the fighting.
Little is known about her personal views on Syria and whether she believes intervention is either possible or desirable. She has never publicly balked as the White House repeated its insistence on staying out, even in the face of evidence the Assad regime is using chemical weapons.
In her writing and thinking, Ms Power often divides US officials into those who pressed for action and those who found reasons not to get involved. As a member of the president's cabinet and America's representative to the United Nations, she may well have to decide into which category she falls.
Telegraph Media Group Limited