On 22 April enemy activity across the whole front suddenly increased and the U.N. advance halted abruptly. The expected spring offensive was at hand.
Following a four-hour artillery bombardment, three Chinese Communist armies attacked the U.N. line in the evening hours of 22 April. The main attack was against the U.S. I and IX Corps in the Seoul sector, coupled with a secondary thrust in the central Yonch'on-Hwach'on area and a p w h in the seat near Inje. U.N. lines held firm except in the IX Corps central sector, where ROK units were forced back in confusion. With the line broken, Van Fleet ordered the I and IX Corps to withdraw through a series of delaying positions to Line KANSAS, thus giving up the ground gained in recent U.N. offensives. When the enemy cut the Seoul-Kaesong highway on 26 April, Van Fleet withdrew the IX Corps back to the Hongch'on River.
Meanwhile, in the I Corps area, the enemy crossed the Imjin River on 22 April and drove the ROK unite south of the KANSAS Line on the 23d. On 27 April the enemy outflanked Uijongbu, forcing U.N. units to pulls back to within four miles of Seoul, and also made an unsuccessful attempt to outflank the city to the east. On the east-central front North Koreans captured Inje. By 29 April, however, their drive had been halted. On this date Van Fleet established a new line, designated NONAME-LINE, extending from north of Seoul to Sabangue and thence northeast across the 38th parallel to Taepo-ri on the east coast. Because the major enemy attack had been in the west, Van Fleet reshuffled his units to put more American divisions there. By the end of April, U.N. forces had stopped the enemy short of Seoul and the Han and held a strong, continuous defense line.
As the enemy withdrew to recoup losses, Van Fleet improved his defenses on NO-NAME-LINE and planned an offensive to carry the Eighth Army back to Line KANSAS; but signs of another impending enemy attack led him to postpone it.
On the night of 15-16 May an estimated 21 Chinese divisions, flanked by 3 North Korean divisions in the west and 6 in the east, struck in the central sector against the U.S. X and the ROK III Corps in the Naep'yong-ni-No-dong area. ROK units were again forced back by the swarming columns of Chinese and North Koreans. Once more Van Fleet reshuffled his units, moved in reserves, and laid down a tremendous curtain of artillery fire which exacted heavy casualties and stopped the enemy offensive.
On 17 May the enemy struck down the Pukhan River toward the Han in the western sector, against the I and IX Corps, with a force of about 250,000 men. This attack was also contained after three days of violent action. By 20 May U.N. troops brought the enemy to a standstill, having thus stopped two major offensives in two months.
Van Fleet decided to renew the offensive, so as to give the enemy no chance to gather himself for another counterstrike. On 18 May he opened a series of local attacks. Once more enemy forces pulled back and U.N. forces moved forward against light resistance. Within a few days the I Corps reached the Imjin River north of Munsan-ni and entered Uijongbu and Sinp'al-li. The IX Corps pushed toward Kap'yong, drove the enemy across the Hanch'on River, and moved toward the Hwach'on Reservoir. In the X Corps area the 1st Marine Division attacked Yanggu on 24 May. The 187th RCT headed for Inje, which it captured on the 27th. The Marines were pushing toward the Hwach' on Reservoir and Yanggu. The 7th Division of the I Corps took Hwach'on. By 31 May the U.N. forces scored a significant advance which brought them just about back to the KANSAS Line, and South Korea was virtually cleared of the enemy.
At this point the Joint Chiefs of Staff prescribed that the Eighth Army was not to go beyond the general vicinity of Line KANSAS. The only tactical operations permitted were those necessary to protect itself, to maintain contact, and to harass the enemy. This was the basic pattern of U.N. military operations which was to be followed throughout the remainder of the war.
On 1 June, therefore, Van Fleet ordered his reserve forces to strengthen KANSAS so as to make it virtually impregnable. Meanwhile the I and IX Corps were to continue Operation PILEDRIVER toward Line WYOMING (the bulge north of KANSAS that ran from the Imjin River to points just south of Ch'orwon and Kumhwa and thence southeast). Ch'orwon and Kumhwa were captured on 11 June. Two tank-infantry task fences reached P'yongyang, the northern tip of the Iron Triangle, on 13 June and found it deserted. The dominating high ground north of the city was held by the enemy, however, and U.N. forces withdrew. The Chinese reoccupied P'yongyang on 17 June. Meanwhile the X Corps on the east-central front pushed through mountains toward its sector of the KANSAS Line, which extended over a series of ridges from the Hwach'on Reservoir northeastward to the lower lip of the "Punchbowl," an aptly named circular depression north of Inje. Thus by mid-June the Eighth Army had attained the principal terrain objectives of Operation PILEDRIVER. Action for the rest of the month was confined to developing the KANSAS and WYOMING Lines, and to patrolling and local clashes.