Description A reconnaissance in force by elements of the 1st Cavalry Division on 22 January revealed that the enemy had withdrawn from frontline positions. The task force returned after having met little resistance. Ridgway then scheduled a larger reconnaissance in force, Operation THUNDERBOLT, with each Corps using one U.S. division and one ROK regiment. The operation began on 25 January and advanced slowly and cautiously against light resistance during the rest of the month. U.N. air support destroyed points of resistance and the enemy's lines of communication were subjected to damaging attacks, which kept a large part of his supplies from reaching the front. By 30 January his resistance stiffened and it continued to be vigorous until 9 February. Then it abruptly gave way. By 10 February U.N. forces secured Inch'on and Kimpo airfield, and the U.S. I Corps closed up to the south bank of the Han River.
On the central front, U.N. armored patrols reached the deserted city of Wonju and elements of the X Corps captured Hoengsong on 2 February against light resistance. On 5 February the X Corps began Operation ROUNDUP, a plan calling for ROK units of the Corps to disrupt the regrouping of North Korean forces south of the town of Hongch'on. On the second day of the attack the ROK units met stiffening resistance, and pressure on the X Corps increased steadily as signs pointed to a large enemy buildup on its front. On the night of 11-12 February, Chinese Communist forces struck the ROK divisions north of Hoengsong and made immediate penetrations which forced the ROK troops to fall back rapidly. U.N. troops withdrew south toward Wonju and abandoned Hoengsong on 13 February. On this same day enemy forces struck at Chip'yong-ni, a road junction and key point of the central zone. The U.S. 23d Infantry Regiment and the French Battalion, forming a defensive perimeter around the town, held off a force of three Chinese Communist divisions for three days before enemy pressure melted away. Meanwhile elements of the U.S. 7th Division and ROK units formed a defensive line north of Chech'on, to check a strong enemy force attacking northeast of Wonju.
In the west the U.S. I and IX Corps were gradually taking all ground in the zones up to the Han River, except for a sizeable enemy foothold south of the Han in an area between Seoul and Yangp'yong. On the night of 13-14 February the enemy launched a powerful counterattack from this area toward Suwon, but his effort was quickly contained with heavy losses to his troops. Meanwhile areas far to the south were being harassed by guerrilla and remnants of North Korean troops. U.N. counteractions succeeded in reducing these forces to about 18,000 by the end of February.
On 18 February combat patrols confirmed a report of the IX Corps that enemy forces along the entire central front were withdrawing. Thereupon Ridgway ordered the IX Corps to move forward, which it did against light scattered resistance. By 19 February the initiative all along the front had passed into U.N. hands.
Ridgway was determined to give the North Koreans and Chinese Communists neither rest nor opportunity to reorganize. On 21 February he launched a general advance (Operation KIILER) by the U.S. IX and X Corps to deny important positions to the enemy and to destroy as many enemy troops as could be found. The objective was a line running eastward from Yangp'yong to the Han River east of Seoul, thence to points north of Chip'yong-ni and Hwangsong-ni, and thence eastward so as to secure the east-west portion of the Wonju-Kangnung road between Wonju and Pangnimni.
Advances in both Corps zones were slow and unspectacular. The spring thaw and heavy rains caused swollen streams and deep mud which greatly hampered military operations. By 28 February the U.N. forces advanced to their assigned objectives, and the Communist foothold south of the Han collapsed. By 1 March the entire Eighth Army line was relatively stable.
Although the Eighth Army had attained its geographical objectives by 1 March, a large part of the enemy had succeeded in withdrawing during the bad weather which had disrupted Allied road and rail movement. With approval by MacArthur, Ridgway planned to continue the attack northward in the central and eastern sectors with Operation RIPPER, to seize Hongch'on and Ch'unch'on and a line designated IDAHO just south of the 38th parallel.
RIPPER began on 7 March 1951. After overcoming initial resistance, the IX Corps reached the first phase line on 11 March and began the attack to the second phase line on the 14th. U.N. patrols moved into the deserted city of Seoul on the night of 14-15 March, marking the fourth time that the capital had changed hands. In the X Corps zone, terrain rather than hostile forces proved to be the greatest obstacle; but despite the enemy and natural obstacles Operation RIPPER ground forward. In the east, ROK units were ordered to destroy the remnants of a North Korean division which had infiltrated southward in January. By 17 March, with this threat eliminated, the ROK forces had moved to Line IDAHO. UN forces entered Ch'unch'on, an enemy supply and communications center, on 19 March.
On 23 March the 187th Airborne RCT dropped at Munsan-ni, about 20 miles northwest of Seoul, to trap enemy troops fleeing northward; but because of the rapid enemy withdrawal it failed to achieve its purpose.
By the last of March Ridgway's forces had fought their way generally to the 38th parallel in position along line IDAHO. Again the U.N. Command was faced with the problem of crossing the parallel into North Korea.
Ridgway, with the approval of President Truman and MacArthur, elected to continue the advance, with the hope of achieving maximum destruction of enemy forces. U.N. commanders made their plans to advance with the knowledge that the enemy was engaged in a full-scale buildup of troops and materiel for his expected spring offensive.
On 5 April Ridgway opened Operation RUGGED, a general advance toward a new objective line called KANSAS. This line, running along the commanding ground north of the 38th parallel, was 115 miles long, including 14 miles of tidal water on the left flank and the 10-mile water barrier of the Hwach'on Dam in the center. By 9 April, the U.S. I and IX Corps and the ROK I Corps on the east coast had reached Line KANSAS, and the U.S. X and ROK III Corps in the central and central-east sectors were drawing up to it. The I and IX Corps continued to advance, attacking Ch'orwon, with the intention of seizing a line designated UTAH, an outward bulge of KANSAS, so as to be in a position to strike at the ''Iron Triangle."
On 11 April President Truman relieved General MacArthur of all his commands because of differences over national policy and military strategy, and replaced him with General Ridgway. Lt. Gen. James A. Van Fleet assumed command of the Eighth Army on 14 April, replacing Ridgway.
Meanwhile U.N. forces continued to edge forward. The Hwach'on Dam was taken on 16 April. On the east coast South Korean forces captured Taep'o-ri. Other ROK troops north of Seoul sent patrols across the Imjin River and far to the northeast. By 17 April U.N. units could not make contact with the enemy, and thereafter the general advance toward Line UTAH was virtually unopposed. Even as it continued, however, evidences of enemy preparations for a counterattack were apparent to the Eighth Army Command. By 19 April all U.S. I and IX Corps units were in positions Along Line UTAH, preparing for an advance to a new Line WYOMING.