OTTAWA -- The day he was alleged to have killed a wounded, unarmed Taliban fighter, Capt. Robert Semrau watched in dismay as stoned Afghan soldiers passed around "King Kong" marijuana joints and carried their rifles like baseball bats.

The former Canadian infantry officer, who was at the centre of a national debate over mercy killing in war, has broken his silence in a book that paints a stark, searing portrait of the chaos in the Afghan war.

An advance copy of the book was obtained by The Canadian Press.

Throughout his trial for second-degree murder and in the aftermath of his dismissal from the military, Semrau has been silent about what happened on Oct. 19, 2008, following a horrific firefight.

And anyone looking for a tabloid-type revelation, an explanation -- or even contrition -- in his book, The Taliban Don't Wave, will be disappointed.

In passages devoid of sentiment and reflection, Semrau recounts the events leading to the discovery of the wounded insurgent in an almost machine gun-like narrative.

The dying enemy fighter had "a small, fist-like hole in his stomach, with a partially severed foot and an injured knee," he writes.

Some of the Afghan soldiers he was mentoring debated whether the man was dead.

"Captain Shafiq Ullah said the man was torn apart, had lost all of his blood in a nearby stream, and was ninety per cent dead," Semrau writes.

"And although they differed in their testimony as to the manner and what was said before the incident, two witnesses basically agreed that I had shot the insurgent two times, in what was later dubbed by the international press as a mercy killing."

His narrative sticks carefully to the public record laid down during his 2010 court martial for second-degree murder and attempted murder, charges of which he was acquitted.

He adds nothing about his motivation and defends only his silence.

"As a Canadian citizen, I had the right to remain silent during my trial. I could not be forced to testify," Semrau wrote.

"I chose to remain silent during my murder trial, and I never gave testimony on the stand, nor did I make a statement for the police. The truth of that moment will always be between me and the insurgent."

About 90 per cent of the book recounts his harrowing 2008 tour of Afghanistan as an officer helping to train often dishevelled, disorganized and diffident Afghan troops in the field.

His recollection of the day leading up to the killing was stark and terrifying.

He describes a dizzying battle, where the confusion of a uniformless guerrilla war was compounded by Afghan government intelligence agents mixing in with Taliban ahead of unit, while insurgents planted bombs and booby trap behind them.

Canadian soldiers mentoring Afghan units had been called away from the fight in Kandahar to help the besieged British Army in Lashkar Gah, the capital of neighbouring Helmand province.

He wrote of how some Afghan troops not with his unit showed up the first morning of the battle dishevelled, barefoot with their boots slung over their shoulders and sporting long beards.

"Yep, they were totally, inexcusably high," Semrau recounted.

"They began passing around what could only be described as King Kong joints and started puffing away, their glassy eyes not really taking anything onboard (but they did like to smile a lot and giggle to themselves)."

By that point in the war, Ottawa and NATO had embraced the notion that training a competent army and police force was the only way out of Afghanistan.

Semrau's chronicle stands in sharp contrast to the picture both the Harper government and the army tried to paint of their Afghan allies throughout that time.

Instead of fearless, wily warriors, Semrau found himself saddled with ill-disciplined troops.

With a nearby unit pinned down in crushing ambush in the hours before the fateful shooting, Semrau argued with the Afghan officer whom he was mentoring.

Captain Shafiq Ullah had refused to mount a rescue mission to extract the trapped soldiers, a siege that was only broken by the rattle of a heavy machine-gun of an American AH-64 Apache.

The attack helicopter left a bloody scene of devastation behind, including the wounded Taliban.

In convicting and sentencing Semrau on the lesser charge of disgraceful conduct, the judge, Lt.-Col. Jean-Guy Perron said: "Shooting a wounded, unarmed insurgent is so fundamentally contrary to our values, doctrine and training that it is shockingly unacceptable behaviour. You made a decision that will cast a shadow on you for the rest of your life."

Only the epilogue is devoted to the charges against him and the heart-rending episodes associated with the trial and the possibility of facing jail time.

Semrau acknowledged he took issue with some of the testimony against him, but never said what he found objectionable about the accounts given to the court.

"At least in combat, I could act and react, but in court, I couldn't jump to my feet and shout 'That's a damn lie!"' he writes.

There was an outpouring of support for him in the aftermath of the charges, including a high-profile Facebook campaign and accusations in some columns that Semrau was being made a scapegoat because of the army's fear of another Somalia-type scandal.

The torture and murder of a Somali teen by soldiers of the disbanded airborne regiment in the 1990s left a indelible mark on the military.

Semrau does not complain about his fate in the book, nor criticize the institution that cast him out.

"Looking back on it all now, with hindsight and more clarity than I had at the time, I am amazed that I survived with a shred of sanity," he writes.

"But self-pity and despair were never options for me, because my wife and daughters were counting on me, and I wasn't about to fail them."

He said it "broke his heart" to be kicked out of the military.

Semrau leaves the morality of what he did for others to debate and decide.