The two marines deny murdering Fergal Caraher, a 20-year-old member of Sinn Fein, and attempting to murder his brother, Micheal, 23, at the village of Cullyhanna, South Armagh, in December 1990.
Both men were shot in a car near a checkpoint. The accused are Lance Corporal Richard Elkington, 23, and Private Andrew Callaghan, 21, both members of 45 Commando, who were among 13 soldiers on patrol in the village.
Opening the prosecution case, a crown lawyer told the Northern Ireland Lord Chief Justice, Sir Brian Hutton, that as the Caraher brothers attempted to drive from a pub car park, L/Cpl Elkington smashed the driver's window with his rifle and opened fire on the car, ordering Pte Callaghan to do likewise.
The court was told that L/Cpl Elkington later told police he had fired nine aimed shots at the driver, believing that a third soldier was being carried away on the bonnet of the car.
Pte Callaghan had said he fired 12 'well-aimed shots' because he feared for the life of the third Marine, whom he could not see.
When the car finally stopped, Fergal Caraher was found to be dead and his brother seriously injured. Counsel said the Marines, on their own admission, had no lawful justification for firing on the car.
He also said forensic investigation of the shooting had been hampered because other soldiers had collected spent bullet cases instead of preserving the scene for police scenes-of-crime officers.
The trial is expected to last between three and six weeks.