On 7 August the KPA began a series of assaults against the UN Pusan defensive perimeter....
Despite initial successes, by 14 September all the KPA assaults had been repulsed. Pusan was secure. General MacArthur had decided in July that he would counter the communist invasion of South Korea with an amphibious landing at Inchon on the west coast, recapturing Seoul and cutting off the KPA army around Pusan. This plan was designated Operation Chromite. Meanwhile, it would be the task of US 8th Army and ROK units at Pusan to break out and exploit the Inchon landings. Chromite occurred on 15 September and was a complete success.
On 16 September UN forces at Pusan began the breakout. Within a week the KPA defences collapsed. US armoured task forces followed by infantry then raced northward from Pusan. On 26 September these linked with units from the Inchon landing force at Osan. By 27 September US Marines of X Corps had overcome the KPA defenders of Seoul. The South Korean capital was liberated. Now cut off from the north, KPA units disintegrated. They fled northward, abandoning vast quantities of equipment and stores. Eventually only 30,000 of the 130,000 KPA invasion force escaped. Pursuing ROK units crossed the 38th parallel into North Korea followed by UN forces. On 19 October the North Korean capital of Pyongyang was captured. UN troops continued moving north dealing with resistance from what remained of the KPA.
3RAR as part of the British Commonwealth Brigade fought its first actions of the war at this time and eventually reached as far north as Chongju. ROK forces captured the ports of Wonsan and Iwon and Units of US X Corps landed at both on 25 and 29 October. US 8th Army continued to advance north while US X Corps move inland and advanced to the north east. On 27 October ROK units reached the Yalu River. The same day, the first of a series of successful attacks and ambushes by Chinese troops on ROK and US units occurred. This was the first concrete evidence that China had entered the war. Chinese deserters confirmed that there were large numbers of Chinese troops hidden in the mountains and poised to attack. The stage was set for a dramatic reversal of fortune for the UN forces in Korea.