In late April, the PVA launched a major counterattack. 486,000 PVA troops assaulted I Corps and IX Corps' sector of the lines. Most of the UN forces were able to hold their ground, but the PVA broke through at Kapyong, where the ROK 6th Division was destroyed by the PVA 13th Army Corps, which penetrated the line and threatened to encircle the American divisions to the west. The 1st Marine Division and 27th British Commonwealth Brigade were able to drive the 13th Army Corps back while the 24th and 25th Divisions withdrew on 25 April. The line was pushed back to Seoul but managed to hold. A second offensive the next month was similarly unsuccessful, as PVA and KPA forces suffered heavy casualties but were unable to push back the Eighth Army forces. In May-June the UN launched another counteroffensive erasing most of the PVA gains.
