At the end of 2000, James Orbinski taped a note to his fridge in Brussels: "I will not renew! I choose love!"

As he describes in his powerful memoir, An Imperfect Offering: Dispatches from the Medical Frontline, he had spent many years on "the medical frontline" in Afghanistan and Kosovo, Sudan and Somalia, and as an agonised witness to the Rwandan genocide of 1994. He served for three years as president of Medecins Sans Frontieres (MSF), which provides emergency medical assistance in more than 70 countries, and even accepted the Nobel Peace Prize on its behalf. Plans were afoot to change the statutes so he could stay on for another term.

He, however, had had enough of the gruelling pace and constant travel, and was keen to lead a normal family life. By 2004, therefore, Orbinski had "found a different way to live my questions" and completed his reinvention as father, husband and academic.

He is now associate professor of medicine and political science at the University of Toronto, a role that combines research, clinical work and teaching medical residents as well as working on a new collaborative PhD programme on global health. He is the star, subject and "narrative thread" of a documentary directed by Patrick Reed, Triage: Dr James Orbinski's Humanitarian Dilemma (2008), which explores the meaning of humanitarianism in the places where he's worked. Similar themes are also central to his book.

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Although Orbinski now spends much less time on the road, he remains an activist, committed to communicating what he has learned in some of the darkest places on Earth. It soon leads him into contentious political territory when he argues for "a complete decoupling of the military and humanitarian and a complete respect for the independence of humanitarian organisations".

He observes: "Only MSF and the Red Cross hold on to and struggle to maintain that conception of independent and impartial humanitarian space. Others gravitate towards what is called 'integrated coherence', with military and political and humanitarian forces working together to a common end. That's a fantasy that it is useful to imbibe if you're waging war in Afghanistan or Iraq, but what if the Nato action fails? If you're seen to be part of it, as many non-governmental organisations are under the strategy of 'coherence', you're tarred with the same brush and put yourself in a position where you are unable to act independently to help the people who need it." As Orbinski indicates in An Imperfect Offering, this principle created a particularly acute dilemma at the time of the Nato intervention in Kosovo: "If we provided moral cover for a Nato-led 'humanitarian war' today, there would be no turning back to a morally coherent claim for an independent humanitarian space tomorrow."

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Humanitarian interventions offer great photo opportunities and chances for macho posturing. A colonel in the US Marines announced to the Somalians in 1992: "We'll be taking the aid workers out there, and if any of them bad boys come out of the bushes, they'd better be ready for one hell of a significant emotional experience." Yet Orbinski believes that such humanitarianism is largely a facade, citing the views of the historian Samuel Moyn that since 9/11 the US has pursued "low-minded imperial ambitions in high-minded humanitarian tones".

At the heart of all this is the Rwandan genocide, which has clearly defined Orbinski's life as well as much of the debate on these issues. Asked if it could have been prevented, he dismisses all academic caveats and qualifications.

"There's absolutely no question," he says. "Remember that there was already a UN peacekeeping force on the ground. When the Belgian infantry, the key fighting force, was withdrawn, that effectively emasculated it. Remember, too, that thousands of the best, battle-experienced troops in the world, European and American, were flown into the region during the genocide to extract foreign nationals.

"In the first place, it was possible to deploy troops - because they did deploy troops. Secondly, it was possible to deploy troops in a manner that actively and effectively deterred killing while they were on the ground.

"Once the genocide was over, there was a massive deployment of troops - thousands and thousands and thousands - into Zaire, literally half a kilometre away from where the genocide had been taking place. There are several studies by competent military analysts that have concluded that a small force of 5,000 troops deployed into Rwanda to back up the UN force on the ground would have stopped the genocide. So there's absolutely no question, none whatsoever." Although he is careful to stress that "the main responsibility for the genocide was in Rwanda, the criminal politics inside Rwanda", Orbinski is adamant that this in no way lets the West off the hook. "Once the genocide had started, it's very clear that American, French and Belgian interests were exercised in a manner that pursued foreign policy knowingly through a genocide. They knew it was going to happen, they knew it was happening and they chose, through their actions, essentially to obstruct any viable effort to deploy troops through the UN."

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Even the resolution, Orbinski reports, left a decidedly sour taste: "The eventual UN/French-led intervention was too little, too late, and barely more than a deception that allowed those who had committed the genocide to escape into Zaire."

The burden of such knowledge, not surprisingly, led to a crisis in his own life. "For 18 months afterwards," he writes, "I existed in a kind of netherworld of confusion, trying to sidestep memories that could impose themselves at any time." Although he studied for a masters in medical epidemiology and discovered that "something about the logic of statistics calmed me", he remained "possessed by what I had seen in Rwanda, trapped between rage and despair".

Orbinski remains focused and earnest, with very little small talk. He is past getting upset by the ordinary irritations of life ("I don't sweat the small stuff, it just doesn't worry me") and he no longer feels frustrated that other people don't see things the same way. Refusing to despair, he tries to act on the principle that "it is the pursuit of something better that makes something better possible, so you have to pursue it, you have to engage and participate". Much of his academic work represents a form of such engagement.

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His research focuses on "developing an integrated system for the delivery of care to people with HIV and tuberculosis in the developing world" through "scientifically tested algorithms that enable poorly educated clinical officers, nursing assistants or community health workers to diagnose, manage and treat those diseases". The hybrid academic NGO he co-founded, Dignitas International, now has 20,000 patients under care in Malawi, a country with only 100 public-sector doctors serving a population of 12 million. The goal is "to maximise the effectiveness of available services, using the resources of communities, decentralising clinics and care to small remote villages".

In other areas, too, such as widening access to essential medicines, Orbinski sees signs of genuine hope and progress. Freely admitting that his core message is simple, he continues the struggle to get it across, not least to some of his colleagues.

"The dominant perspective sees humanitarianism as a means to an end; there's always a utilitarian or consequentialist calculation. What's the benefit, the outcome, the good of saving this person's life when you know that they could die tomorrow? What's the point of that? The point is to help the person in very difficult circumstances - that's the point, period. It's not to solve all the causes of that person's condition, but to respond to that person's situation in a human and practical way, so they can participate in their own destiny, live their life in their own way. That's an end in itself," Orbinski says.

"It's not a new idea but is sometimes considered a radical idea - which I find quite sad. It's sometimes surprising to political scientists, to military strategists, to scholars of international relations."

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An Imperfect Offering: Dispatches from the Medical Frontline is published by Rider/Ebury Books.