An investigation has been launched into two police officers and a special constable who were caught on camera attacking a soldier.
CCTV footage shows Lance Corporal Mark Aspinall being punched on the floor by one officer while under restraint from two others.
Mr Aspinall, who has served in Iraq and Afghanistan, was set upon after a night out in his home town of Wigan, Lancashire, in July. He was then charged and convicted on two counts of attacking police officers - until last week when he won an appeal at Liverpool Crown Court.
The turnaround has sparked an investigation with one officer's duties already restricted.
Terry Sweeney, Assistant Chief Constable of Greater Manchester Police, said: "Greater Manchester Police's professional standards branch is investigating the conduct of the officers on the Wigan division.
"One officer has had his duties restricted and other two are being investigated. The matter has been voluntarily referred to the Independent Police Complaints Commission, but an independent complaint has also been made."
An Independent Police Complaints Commission spokesman said the investigation would be conducted under the watchdog's "management, direction and control".
The CCTV begins with Mr Aspinall - who admits he was drunk - standing unsteadily in the street and calling to the officers.
The trio run 10ft across the road at him and he falls over but jumps up. An officer is seen rugby tackling the 24-year-old who is pinned down by two officers while the third punches him several times.
Mr Aspinall, who returned from Afghanistan in February and was working his notice in the Army at the time of the attack, told the Sunday Mirror: "I went into the Army thinking this country was worth fighting for. I put my life on the line every day in Afghanistan, so to come back and be treated like this for no reason was just so depressing."
