29 July 2013 Last updated at 09:04 ET 
Wesley Clutterbuck, 19, hit his head on the ground after the attack in Fleece Street, Rochdale, on 30 June. He died in hospital the following day.
Reece Kay, of Waithlands Road, Rochdale, admitted manslaughter at a hearing on 4 July at Manchester Crown Court.
He will serve the sentence at a young offender institution.
Elizabeth Reed, Branch Crown Prosecutor said: “A young man, who was loyally serving our country, has lost his life in this senseless and unprovoked attack which clearly demonstrates the cost of just one punch.
“Kay ran towards the victim, who was a complete stranger to him, with a clenched fist and punched him once to the head causing him to fall over and hit his head on the pavement.
“He then turned and ran from the scene showing complete disregard to the harm he had caused.
“Nevertheless, he later admitted the crime and as a result we have been able to bring him to justice promptly, less than a month after the incident took place.”
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