Operation Killer was the start of the second major counter offensive launched by United Nations Command (UN) forces against the Chinese Communist People's Volunteer Army (PVA) and the North Korean Army (KPA) during the Korean War between 20 February and 6 March 1951. The offensive was formulated by General Matthew Ridgway with the goal of annihilating enemy forces south of a line designated the Arizona Line. The operation was immediately followed by Operation Ripper.