The newly arrived United Nations team looked like people suffering from shell-shock even before they had heard their first real explosion. Picking their way through the dingy lobby at the Sheraton Ishtar hotel in Baghdad, they fought past inquisitive journalists and supplicant Iraqis into the chaos and 50C heat of Baghdad.

The city had only recently been captured by American troops, looting was rife, the US authorities were struggling to maintain order and many believed that the job of trying to rebuild the country would eventually fall to these reluctant UN officials.

The prospects were not encouraging, except for the presence of one man: Sergio Vieira de Mello, a veteran diplomat from Brazil, a charmer and womaniser who was credited with helping to rehabilitate more dysfunctional countries than any other UN troubleshooter.

With his permanent tan and an endless supply of finely tailored suits and Johnnie Walker Black Label whisky, if anyone could wrest something positive from the unfolding disaster, the UN envoy could.

Unfortunately for Iraq and the UN, we will never know what he would have achieved during his short stay in Baghdad. The newly constituted al-Qaeda in Iraq, headed by the late Abu Musab al-Zarqawi, had reached the same conclusions about the impact of the UN's representative. They were determined to make an example of Vieira de Mello.

We have now become inured to reports and pictures of suicide bombers killing scores of innocent victims in Iraq, but on August 19, 2003 these attacks were still a rarity. The first major target was the poorly defended UN compound in Baghdad, which shook to the force of a truck bomb loaded with 1,000lb of explosives.

The blast destroyed part of the building and killed 23 people, most of them UN officials. Vieira de Mello's office took the brunt of the explosion, trapping the 55-year-old Brazilian under tonnes of concrete. Badly injured, he held out for three hours before succumbing to his wounds. The UN was thrown into a state of shock and, within weeks, all of its employees were evacuated. The attack shook the organisation's confidence and triggered a crisis from which it has still not quite recovered to this day.

Chasing the Flame is a brilliantly researched biography about an extraordinary man. He sacrificed his marriage and the chance of a quiet bureaucratic life, favoured by so many of his UN colleagues, to serve in the world's hotspots, such as Cambodia, Lebanon, Rwanda and Bosnia. He headed the UN mission to Kosovo and East Timor, ran the organisation's humanitarian and human rights agencies, and was the obvious choice to dispatch to Baghdad. Kofi Annan, the former UN Secretary-General, said after learning of Vieira de Mello's death: “I had only one Sergio.”

Samantha Power does not give the UN martyr a free ride, however. Vieira de Mello may have learnt politics in Paris during the student demonstrations of 1968, when he was badly injured by French riot police - but her book reveals a man who favoured pragmatism over idealism and was quite prepared to charm the Khmer Rouge killers and Serb war criminals if it would help him to achieve his ends. Carina Perelli, a feisty UN colleague who returned to Iraq to organise the country's elections, described him as an encantandor de serpientes, a man capable of charming the most poisonous snakes.

Before going to Baghdad Vieira de Mello held a long, and by all accounts friendly, meeting with President Bush at the White House, and was regarded as Washington's favourite choice as UN envoy. Once he arrived, however, he was kept away from any of the important decisions being made by Paul Bremer, the American viceroy in Baghdad.

“He was like a Victorian parlour maid,” reflected Sir Kieran Prendergast, another UN colleague, who was deeply opposed to the UN mission in Iraq from the start. “Seduced and discarded.”

Chasing the Flame: Sergio Vieira de Mello and the Fight to Save the World, by Samantha Power
Penguin, £25

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