The human cogs of the torture machine seemed as unhappy as their victims. Which meant, I thought as I scribbled in a notebook, ‘There’s no rational explanation for the machine’s existence at all.’

Not least of the problems facing coalition authorities after the fall of Saddam Hussein was the question of ‘de-Ba’athification’. In a country where there was one agent of the state for every twenty civilians, where the five secret police forces were themselves monitored by additional secret police forces, where almost everyone from military generals to primary school teachers were forced into collusion with Ba’athist ideology… where did you draw the line? Where does the forced complicity of the Iraqi barber forced at gunpoint to inform on his or her clients become the conscious evil of the high-ranking believer?

For as the Ba’ath Party psychologist Dr Laith tells us, ‘It was as if I had two or more personalities. I would do my best as an officer with my duties and then I would come home and speak against the regime. All Iraqis have two or more characters.’ American journalist Wendell Steavenson wanted to do for Ba’athism what Hannah Arendt and Robert Lifton had done for Nazism: to understand the perpetrators as well as the victims.

She focuses on Kemal Sachet, a Ba’athist general and military hero who fell in and out of favour under Saddam’s system of capricious evil. Through the character of Sachet, she speaks to his family, his colleagues and his friends, drawing an expansive picture of a people staring at the blood on their trembling hands. We are constantly aware of the backdrop: a traumatised and disintegrating nation pummelled by coalition forces and psychotic terrorists. Steavenson: ‘I always wore a big black tent abaya as disguise in the back of the car, texted my whereabouts to a friend every hour, and took care never to walk down the street.’

Any attempt to understand a perpetrator of evil involves the risk of misunderstanding: to tease out the tiny flickers of humanity inside terrible men, the cheesy filial in-jokes, the annual donations to some orphanage or hospital, the mawkish horror of the SS guard who buys marzipan for his daughter on the way back from a shift at the ovens. Yet although Steavenson writes about Sachet’s personal life, she does not succumb to the slobbering awe that afflicts even radicals when they are faced with undeniable power. In the book she argues with and contradicts her subjects – there is a great passage where she debates Muslim grievances with a supporter of the ‘resistance’ – but her narrative seeks less to understand than to tell a story, never forgetting that context is all.

Yet Sachet seems to have been quite humane by the Ba’ath’s miserable standards, deploring the senseless loss of life caused by Saddam’s invasion of Kuwait, and once helping to clean out a storm drain in his general’s uniform. Indeed, Sachet’s later life appears marked by a series of urges towards good deeds, but Steavenson is sceptical as to whether these were attempts at genuine atonement or a last-minute scramble for a place in heaven. ‘He gave money to the needy,’ Steavenson writes, ‘He thought of Allah and his kingdom of heaven and compensated his guilt with humility. When he held the hand of a frail old man dying in a hospital bed he would say to himself, ‘Ten credits.’ Sachet fears Saddam during his lifetime, but as the grave looms before him he realises that the only dictator that counts is the one in the sky. The title of Steavenson’s book comes from a Koranic verse about the scales of justice on which every soul is weighed. One good or evil deed can cause a decisive swing, even ‘if there be (no more than) the weight of a mustard seed’.

Ultimately, Steavenson’s book is a study of what Kant called ‘moral luck’ and what Stephen King called ‘black serendipity’. Most people in democratic countries will never be in a position where they are complicit with killing and oppression – although haven’t we all met some pompous bully in a position of minor authority, and thought something like: ‘Stalin would have loved you’? But what if you are born under dictatorship? What if you are conscripted to a fascist army? What if the fascist army comes to your village and threaten to shoot your children unless you collaborate? Can you redeem yourself with little acts of kindness and subversion?

This is your descent into the moral swamp of what another Ba’athist doctor calls ‘Yes… But’; ‘What could I do?; ‘ But I helped many, many people!; ‘I suffered also, you know’; and the ultimate trumping, ‘You cannot understand what it is like to live under such a regime!’

A travelogue with the language and scope of a novel, Steavenson’s book will be essential reading for historians studying the political literature of Iraq: a nation that, like the souls of the dead, still hangs in the balance.

The Weight of a Mustard Seed, Wendell Steavenson, Atlantic 2009