Former soldier Chris Henchliffe died after being punched by a young man in an 'act of aggression', a court heard.
Benjamin Edwards, 22, is accused of unlawfully killing 26-year-old Mr Henchliffe, who passed away in hospital nine days after allegedly being attacked outside the Pomegranate Theatre in Corporation Street, Chesterfield, during the early hours of July 3 last year.
Edwards, of Fieldview Place, Chesterfield, denies manslaughter.
His trial, which is expected to last into next week, got underway at Nottingham Crown Court this morning.
After a jury of six men and six women was sworn in, prosecutor Andrew Vout said: "Shortly after 4am that morning (July 3), the defendant punched Chris Henchliffe to the face.
"This blow caused Chris Henchliffe to fall heavily to the floor and strike his head.
"As a result of this, he suffered a catastrophic head injury.
"He never recovered and died nine days later.
"Benjamin Edwards, through lawyers, indicated he punched Chris Henchliffe in reasonable defence of another person.
"If that is right, he is not guilty.
"We, the prosecution, say that Benjamin Edwards didn't punch him in reasonable defence of another person.
"It was an act of aggression"
Mr Vout told the court Mr Henchliffe was out with three women in Moo Bar in Corporation Street on July 3 and spilled some drink over a man.
The prosecutor said a doorman told Mr Henchliffe to leave the bar so he exited 'without argument' and walked across the road.
According to Mr Vout, CCTV footage showed a man called James Wisternoff - who was on a night out with Edwards - join Mr Henchliffe at the entrance of the Pomegranate Theatre.
Mr Wisternoff's sister, Clarissa Wisternoff, a Moo Bar employee, approached the men amid accusations Mr Henchliffe had thrown a drink at her, Mr Vout said.
He added: "Chris Henchliffe was heard to say, 'why are you lying, you b***h?'
"James Wisternoff said, 'that's my sister you're talking to'.
"Chris Henchliffe replied, 'she's f*****g lying."
Mr Vout said the CCTV footage showed Mr Wisternoff throw 'two or possibly three punches towards Chris Henchliffe, causing him to stagger backwards'.
Mr Henchliffe 'raised his arm' towards Mr Wisternoff before a doorman took hold of Mr Henchliffe, Mr Vout added.
The prosecutor told the jury: "At 4.02am, Benjamin Edwards runs across and punches Chris Henchliffe to the head with force, causing him to fall backwards and strike his head on the pavement.
"Then he runs away with James Wisternoff and two others."
On that Sunday afternoon, Mr Vout said, Edwards sent a Facebook message to a friend saying, 'don't report anything about last night. I might be getting sent down'.
The friend, Joseph Bradfield, replied, 'I hope the guy's going to be OK. Consider owning up'.
Edwards was later arrested.
He was originally charged with inflicting grievous bodily harm but, following Mr Henchliffe's death, was charged with manslaughter.
Dad-of-one Mr Henchliffe, who lived on Maynard Road, Chesterfield, served with the Yorkshire Regiment and completed a tour of Afghanistan.
The trial continues.