It was the single, split-second decision of a man at breaking point that has become one of the most contentious issues of the war in Afghanistan.

Sergeant Alexander Blackman, coming to the end of a "hellish" six-month tour of the Helmand province where he had seen friends and comrades killed and their body parts hung up as trophies, killed a dying insurgent with a single shot at point-blank range.

The moment was captured on film and Blackman, a veteran of five tours in Iraq and Afghanistan and described as a "superb soldier", became the only British serviceman known to have been convicted of murder on a foreign battlefield.

Today, after spending more than three years in prison for the offence, five of the most senior judges in Britain quashed his conviction and replaced it with one for manslaughter, recognising the extreme stresses that Blackman faced in the theatre of war.

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At the moment that he pulled the trigger, he and the band of 15 marines under his command had been manning Checkpoint Omar, a remote British outpost in what was dubbed the "most dangerous square mile on Earth".

So treacherous even the padre was banned from visiting, they faced daily attacks from the Taliban and were desperately understaffed in a camp that was a "breeding ground" for mental health problems.

Split into two groups with one led by Blackman - known at his original military trial as Marine A - they had to carry out twice-daily patrols and it is estimated they faced an IED explosion every 16 hours.

Each step was potentially their last and Blackman's wife, Claire, noticed that when he returned home to Somerset he still searched the ground in front of him.

In total, seven soldiers from the 42 Commando Royal Marines died and 45 were seriously wounded during the tour.

The tipping point for Blackman came in in May 2011 when Lieutenant Ollie Augustin, 23, was leading the other half-troop of marines and was blown up and killed instantly with Marine Sam Alexander, who had been awarded the military cross for a previous tour, and their interpreter.

As they made their way back to their temporary camp, the men saw the legs of a fellow soldier hanging from the tree, believed to have belonged to a teenage marine and placed there by the Taliban as a trophy.

Blackman was not present, but had mentored Lieutenant Augustin and was deeply affected by his death. He put his own life on the line to try to make sure others made it back to their families.

The appeal judges had even noted that he "regarded himself as responsible for the members of his troop, particularly those with children (the appellant had none); he therefore undertook more patrols and risks to himself so that his troops could all get home safely".

He and his men later joined the hunt for Highlander Scott McLaren, a 20-year-old soldier who had gone AWOL from his base and was later found dead having been tortured and murdered by insurgents.

Sent to win over the hearts and minds of the Afghan people, they faced the constant threat from those who appeared friendly. In August Blackman had a lucky escape after two grenades were thrown at him as he stood outside the camp.

Feeling abandoned by their superiors, with commanding officer Lieutenant-Colonel Ewen Murchison only visiting Omar twice during the entire tour, the men went "feral", the court heard.

It was against this background, sleep deprived and afraid in the most dangerous place on earth, that Blackman, described by colleagues as a "husk of his former self", fired the shot that made him the only British serviceman convicted of murder in the Afghan war.

On September 15, 2011 a Taliban attack on a British base was stopped when a Apache helicopter hit the insurgents with cannon fire. One was found mortally wounded in a field with a AK-47, two magazines and a hand grenade at his side by Blackman and two of his men.

There was some discussion of giving medical help , with one soldier applying a field dressing, after they had dragged him to a corner of the field where they could not be seen. They told the dying man: "Don't give a f*** about you son".

After saying that he had "passed", Blackman drew his pistol and shot the Afghan, saying: "Shuffle off this mortal coil, you ****."

Blackman continued: "It's nothing you wouldn't do to us. Obviously this doesn't go anywhere, fellas. I've just broke the Geneva Convention".

Unbeknown to Blackman the incident was captured on the helmet camera worn by one of his men. The video was found by British police during the course of a separate investigation in September 2012 and he was arrested.

He later recalled of the joking captured on the film, saying: "When you are facing on a daily basis people trying to kill you, you inject humour as a coping mechanism as otherwise I would say things would be very dark".

Blackman has always maintained that he believed that the man was already dead, and he had broken the Geneva convention by desecrating a corpse.

Despite the criminal courts having jurisdiction to hear his case, it was decided he would be tried by court martial led by the Judge Advocate General and heard by two marine officers, three Royal Naval officers and a Royal Naval warrant officer.

He was convicted of murder in December 2013. He had not been examined by a psychiatrist for signs of combat stress and there had been no mention of a lesser conviction of manslaughter. The two marines who were with him were cleared of any wrongdoing.

As a result of his conviction Blackman was dismissed with disgrace from the Royal Marines after serving with distinction for 15 years and Judge Blackett told him he had provided "ammunition to the terrorists".

It was then that Blackman's fight for justice, led with determination by Claire Blackman, an NHS marketing manager, began.

He has been supported by thousands of his fellow soldiers and former top brass, with some serving soldiers defying a Ministry of Defence ban on attending rallies to support him. Colonel Oliver Lee, who had taken charge of his unit days before the shooting, quit his post in disgust at his treatment.

Claire Blackman, the wife of convicted British Royal marine Alexander Blackman, speaks to the media outside the High Court in London. Photo / AP
Claire Blackman, the wife of convicted British Royal marine Alexander Blackman, speaks to the media outside the High Court in London. Photo / AP

An original appeal against conviction in May 2014 saw the minimum tariff of his life sentence reduced from 10 to eight years.

But his supporters, who claim that he was made a political scapegoat for wider failings in Afghanistan, did not give up and in December last year he was granted a second appeal.

The Criminal Cases Review Commission said that there was a realistic prospect of overturning his conviction because of further psychological evidence, the failings of the original judge to offer a verdict of manslaughter, and failings of his original legal team.

Finally, in February this year prosecutors conceded that he was suffering from some sort of mental illness.

Three leading psychiatrists told the Court of Appeal that he was suffering from "adjustment disorder" which impaired his ability to make rational decisions and which had "numbed his moral compass".

Professor Neil Greenberg, a military psychiatrist, told the High Court hearing: "There isn't any such thing as a Rambo-type, Arnold Schwarzenegger soldier who can face all sorts of stressors and appear to be invulnerable. That sort of person only exists in the cinema."

Despite being deemed so trustworthy when on bail before his original conviction he was allowed to instruct Royal Marines with live ammunition, he was denied bail as he awaited the outcome of the appeal.

He has been described as a model prisoner in HMP Erlestoke in Wiltshire, teaching other prisoners maths and volleyball.

Because of his tours and subsequent imprisonment he and his wife have now spent more of their seven years of married life apart than together.

Today, Appeal Court judges overturned his murder conviction saying that the stresses of war had "substantially impaired his ability to form a rational judgment".

With his reputation in tatters and unable to wear the green beret of his corps, Blackman must now wait to find out how much longer he spends behind bars before returning home to his wife and a job as a physical training instructor.

In a rare interview from behind bars, he had admitted: "Do I want to become a personal trainer when I'm released? Not really.

"But I have to face the fact that unless my conviction is quashed, I will leave here with a criminal record. And when you are in that position you need to have as many strings to your bow as possible."