In 2502, the UNSC, specifically Project: ORION, sent a young, but capable Corporal Avery Johnson to assassinate Jerald Mulkey Ander, a leader of the People's Occupation Government. The UNSC sent Johnson to Harvest, which at the time was a new colony, to assassinate him. This operation was also Johnson's final test for Naval Special Warfare Sniper School. 
Johnson acquired a position a fair distance from his target and set up a M99 Special Application Scoped Rifle. Along with Johnson there was another UNSC Marine, presumably his spotter for making calculations for wind and distance.
When Ander's truck passed by Johnson took the shot and successfully killed Ander, ending the "Secessionist Union".