United Nations: Operation Killer was a military operation that marked the start of the second major counter offensive which United Nations Command (UN) forces launched against the Chinese Communist People's Volunteer Army (PVA) and the North Korean Army (KPA) during the Korean War.
[3]: 306 The west flank of the advance was to be adequately protected by US I Corps and 24th Infantry Division positions along the lower bank of the Han River.
If ROK III Corps commander Major General Yu Jae-hung was unable to develop continuous defenses above the road, he was to at least ensure possession of Gangneung.
[3]: 306 A week prior to Operation Killer, PVA forces broke up X Corps' Roundup advance and threatened to strike deep through the Han River valley.
The Marines were to relieve the US 2nd Infantry Division and 187th Airborne Regimental Combat Team (187th RCT) in the Wonju area, which on the 21st would fall within the zone of IX Corps.
At the left of the Corps' zone, the ROK 3rd Infantry Division was to clear enemy forces from an area which narrowed to a point on Route 20.
If the timing was right, General Ruffner's forces could hammer enemy units against an anvil provided by 7th Infantry Division troops driving west over Route 20.
Together, the unseasonable rain and warmer temperatures transformers the rivers and streams into courses of deep, fast water filled with floating ice.
Ahead of the advance, the PVA/KPA concentrated on evacuating the salient, leaving behind only scattered forces to fight the occasional but strong delaying actions.
From west to east, the IX and X Corps front on the last day of the month traced a shallow concave arc from positions 5 miles (8.0 km) above Chipyong-ni, along high ground overlooking Hoengsong and Route 20 from the south, to the high hills 4 miles (6.4 km) north of the Routes 20- 60 junction.
In the IX Corps' zone, the 1st Marine Division cleared Hoengsong with little opposition on 2 March en route to Arizona objectives 3 miles (4.8 km) north of town.
By evening of 6 March all IX Corps' assault units had established positions near or slightly above the Arizona Line, with the final advances encountering no resistance at all.
The X Corps' units met stiff opposition over the first five days of March, particularly the 2nd Infantry Division as it attempted to occupy the high ground just above Route 20.
But from the outset it had become steadily clearer that the primary objective of Operation Killer of destroying all PVA/KPA forces below the Arizona Line would be only partially achieved.
The PVA/KPA forces' head start in withdrawing, their disinclination to take a defensive stand below the objective line other than in spotty delaying actions and difficulties in negotiating the ground had prevented any other result.