The Royal Marine and two other soldiers, who cannot be named for legal reasons, are charged with murdering the man on Sept 15, 2011. The three men deny the charges.
Prosecutor David Perry said today the incident took place after a military base in Afghanistan's Helmand Province was attacked by two insurgents. He said the three Marines discovered the injured Afghan in a field, moved him to a sheltered area and shot him.
Perry said one soldier fired the gun, while the other two "encouraged and assisted" the killing.