Operation Ripper, also known as the Fourth Battle of Seoul, was a United Nations (UN) military operation conceived by the US Eighth Army, General Matthew Ridgway, during the Korean War. The operation was intended to destroy as much as possible of the Chinese People's Volunteer Army (PVA) and Korean People's Army (KPA) forces around Seoul and the towns of Hongch'on, 50 miles (80 km) east of Seoul, and Chuncheon, 15 miles (24 km) further north. The operation also aimed to bring UN troops to the 38th Parallel. It followed upon the heels of Operation Killer, an eight-day UN offensive that concluded February 28, to push PVA/KPA forces north of the Han River. The operation was launched on 6 March 1951 with US I Corps and IX Corps on the west near Seoul and Hoengsong and US X Corps and Republic of Korea Army (ROK) III Corps in the east, to reach the Idaho Line, an arc with its apex just south of the 38th Parallel in South Korea.

Operation Ripper was preceded by the largest artillery bombardment of the Korean War. In the middle, the US 25th Infantry Division quickly crossed the Han and established a bridgehead. Further to the east, IX Corps reached its first phase line on 11 March. Three days later the advance proceeded to the next phase line. During the night of 14–15 March, elements of the ROK 1st Infantry Division and the US 3rd Infantry Division liberated Seoul, marking the fourth and last time the capital changed hands since June 1950. The PVA/KPA forces were compelled to abandon it when the UN approach to the east of the city threatened them with encirclement.

Following the recapture of Seoul the PVA/KPA forces retreated northward, conducting skillful delaying actions that utilized the rugged, muddy terrain to maximum advantage, particularly in the mountainous US X Corps sector. Despite such obstacles, Operation Ripper pressed on throughout March. In the mountainous central region, US IX and US X Corps pushed forward methodically, IX Corps against light opposition and X Corps against staunch enemy defenses. Hongch'on was taken on the 15th and Chuncheon secured on the 22nd. The capture of Chuncheon was the last major ground objective of Operation Ripper.

UN forces had advanced north an average of 30 miles (48 km) from their start lines. However, while the Eighth Army had occupied their principal geographic objectives, the goal of destroying PVA forces and equipment had again proved elusive. More often than not, the PVA/KPA forces withdrew before they suffered extensive damage. Chuncheon, a major PVA/KPA supply hub, was empty by the time UN forces finally occupied it. As the UN troops ground forward, they were constantly descending sharp slopes or ascending steep heights to attack enemy positions that were sometimes above the clouds. By the end of March, US forces reached the 38th Parallel.

As Operation Killer had entered its final week with limited results already predictable, Ridgway published plans for another attack, again with the main effort in his central zone, but with all units on the Eighth Army front involved. As in Operation Killer, the primary purposes of the attack, which Ridgway called Operation Ripper, were to destroy PVA/KPA forces and equipment and to interdict their attempts to organize an offensive. A secondary purpose was to outflank Seoul and the area north of the city as far as the Imjin River. Aware of UN commander General Douglas MacArthur’s interest in recapturing Seoul, but preferring to avoid a direct assault across the Han River into the capital (although plans had been prepared for such an operation), Ridgway hoped to gain a position from which he could take Seoul and the ground to the north by a flanking attack from the east or simply by posing a threat that would induce enemy forces to withdraw from that area.

Ridgway published the Ripper plan on 1 March but deferred setting an opening date because of forward area supply shortages, particularly in food, petroleum products, and ammunition. The shortages resulted partially from conscious efforts during February, especially during the Chinese Fourth Phase Offensive in mid-February, to hold down stockpiles in forward dumps as a hedge against losses through forced abandonment or destruction. In addition, as stocks were expended in the Killer advance, the damage to roads, rail lines, bridges, and tunnels caused by the rains and melting ice and snow severely hampered resupply. Before setting a date for the operation Ridgway wanted a five-day level of supplies established at all forward points. The best estimate at the beginning of March was that this level could be reached in about five days.

Regardless of success in meeting this logistical requirement, Ridgway intended to cancel the operation if in the time taken to raise forward supply levels new intelligence disclosed clear evidence of an imminent PVA/KPA attack. Neither the capture of new ground nor the retention of ground currently held were essential features of Eighth Army operations as Ridgway conceived them. "Terrain," he maintained, "is merely an instrument... for the accomplishment of the mission here," that of inflicting maximum losses on the PVA/KPA at minimum cost while maintaining major units intact. Intelligence indicated that those forces giving ground before the Killer advance in the IX and X Corps' zones were moving into defensive positions just above the Arizona Line. Eighth Army intelligence officer Colonel Tarkenton, believed these forces would tie in with the existing PVA/KPA front tracing the north bank of the Han River in the west and passing through the ridges above Route 20 in the east. Lending support to this judgment, the PVA 39th Army had moved up on the line in front of IX Corps, and the KPA III Corps, less its 3rd Infantry Division, had entered the line before X Corps. Thus, as of 1 March six PVA armies and four KPA corps were arrayed between Seoul and the spine of the Taebaek Mountains. On the 1st, as he had earlier, Tarkenton carried the PVA 37th Army in his enemy order of battle, locating it immediately behind the center of the PVA/KPA front in the vicinity of Chuncheon. In his earlier estimate he tentatively had placed the 43rd Army in the same area, but had since decided that this unit was not in Korea at all. Tarkenton now also had reports that two PVA armies, the 24th and 26th, had moved south from the Hungnam-Wonsan region to a central assembly just above the 38th Parallel north of Chuncheon. Thus three reserve armies might be immediately available for offensive operations in the central region. To add substance to this possibility, agents recently returning from behind enemy lines brought back reports that the PVA/KPA high command at one time had planned to open an offensive on 1 March, then had postponed the opening date to the 15th. During interrogation, recently taken prisoners of war partially substantiated the agent reports by stating that their forces were preparing to launch an offensive in the Eighth Army's central zone early in March. It also now appeared that the KPA VI Corps, one of the units that had withdrawn into Manchuria the past autumn, had returned to Korea and was moving toward the front in the west. At last report the VI Corps, or a part of it, was approaching the 38th Parallel northwest of Seoul and thus was near enough to join an offensive. Tarkenton concluded, however, that although the PVA/KPA high command was preparing an offensive, its opening was not imminent. He reached that conclusion mainly on grounds that the bulk of their reserves were too far north for early employment.

Amid efforts to acquire fuller information on PVA/KPA preparations and plans, Ridgway arranged an amphibious demonstration in the Yellow Sea in an attempt to fix PVA/KPA reserves and to distract their attention from the central zone in which the main Ripper attack was to be made. Minesweepers of Task Force 95 began the demonstration with sweeps along the west coast and into the Taedong River estuary in the vicinity of Chinnamp’o. A cruiser and destroyer contingent followed to bombard pretended landing areas. Troop and cargo ships next left Inchon, steamed part way up the coast, then reversed course. On 5 March the same ships made an ostentatious departure from Inchon to continue the illusion of an impending amphibious landing. In the Sea of Japan, Task Force 95 had placed the Wonsan area under bombardment in February and continued the campaign into March. This bombardment, coupled with the occupation of an offshore island by a small party of South Korean Marines, added to the impression of imminent landing operations. Ridgway had learned that two recently federalized National Guard infantry divisions, the 40th and 45th, were soon to leave the United States for duty in Japan. In an attempt to enlarge the amphibious threat, he proposed to MacArthur that the departure of the divisions be publicized and a deception plan be developed to indicate that the two units would make an amphibious landing in Korea. Extending the idea further, Ridgway also proposed creating the illusion of forthcoming airborne operations by having three replacement increments of 6,000 each put on 82nd Airborne Division patches after arriving in Japan and wear them until they reached Korea. He made this second proposal on the basis of intelligence indicating that the PVA/KPA thought the 82nd was in Japan. Nothing came of either proposal.