Sergeant Alexander Blackman has been convicted for the murder of an injured Afghan fighter
Sergeant Alexander Blackman, 42, from Taunton in Somerset, watched proceedings via video link from prison as his QC outlined his case to five judges in a packed London courtroom.
Blackman's wife Claire and dozens of veterans sat in the public gallery to hear Jonathan Goldberg opening the case on Tuesday, explaining that the "impact" of fresh psychiatric evidence lay at the heart of the appeal.
At the time of the 2011 incident Blackman was serving in Helmand province with Plymouth-based 42 Commando.
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Only those who have been on the front line can know what it is really like
Jonathan Goldberg QC
Mr Goldberg told the Court Martial Appeal Court that the conditions were "austere" and a "breeding ground" for mental health problems.
Describing the Taliban as "ruthless and cunning", he said there were shooting incidents on a regular basis, sometimes daily.
Blackman had "endured" the loss of a young company officer "with whom he had been on very close terms" and had mentored.
Sgt Blackman was serving in Helmand province with Plymouth-based 42 Commando
Mr Goldberg said Blackman himself was almost killed in a grenade attack. That must, he said, have "left mental scars".
He told the judges: "Only those who have been on the front line can know what it is really like."
It was a recognised feature of mental illness that many people do not "recognise symptoms in themselves".
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The QC added: "Mr Blackman's nature is to be very reserved. He is a sort of John Wayne character."
Blackman shot the insurgent in the chest at close range with a 9mm pistol
Blackman's case has been referred to the court by the Criminal Cases Review Commission (CCRC), the independent body that investigates possible miscarriages of justice.
The CCRC announced it had concluded that a number of new issues, including fresh evidence relating to Blackman's mental state, "raise a real possibility" that the Court Martial Appeal Court "will now quash Mr Blackman's murder conviction".
The conviction challenge is being heard by Lord Chief Justice Lord Thomas, Sir Brian Leveson, Lady Justice Hallett, Mr Justice Openshaw and Mr Justice Sweeney.
Blackman was convicted in November 2013 by a court martial in Bulford, Wiltshire, and sentenced to life with a minimum term of 10 years.
The marine's case has been referred to the court by the Criminal Cases Review Commission
In May 2014, the Court Martial Appeal Court rejected a conviction challenge, but reduced the minimum term to eight years because of the combat stress disorder he was suffering from at the time of the shooting.
Blackman shot the insurgent, who had been seriously injured in an attack by an Apache helicopter, in the chest at close range with a 9mm pistol before quoting a phrase from Shakespeare as the man convulsed and died in front of him.
He told him: "There you are. Shuffle off this mortal coil, you ****. It's nothing you wouldn't do to us."
He then turned to his comrades and said: "Obviously this doesn't go anywhere, fellas. I just broke the Geneva Convention."
The shooting was captured on a camera mounted on the helmet of another Royal Marine.
Blackman's wife Claire sat in the public gallery to hear Jonathan Goldberg opening the case
During his trial, Blackman, who denied murder and was known at that stage as Marine A, said he believed the victim was already dead and he was taking out his anger on a corpse.
He was "dismissed with disgrace" from the Royal Marines after serving with distinction for 15 years, including tours of Iraq, Afghanistan and Northern Ireland.
One of the grounds of appeal is that the new psychiatric evidence would have provided him with the "partial defence of diminished responsibility".
At the start of the proceedings the judges lifted reporting restrictions which had previously been in place.
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