The second Communist invasion reached its high point on 24 January 1951. From 25 January, however, the UN forces launched a limited counteroffensive, which crept slowly northward reaching the southern outskirts of Seoul by 10 February. Operation Ripper, which began on 7 March, was designed to inflict casualties on the enemy by means of heavy airborne and ground bombardment. Seoul was liberated a week later, the city changing hands for the fourth and last time during the course of the war. By 31 March UN forces were approximately back along the line of the 38th parallel. The Chinese were preparing for a new offensive, and General Ridgway, who had been appointed Supreme Commander in Korea on 11 April, continued attacks on the so-called Iron Triangle in the centre of the Communist line, which served as a base area to supply the planned Chinese offensive. This began in April with up to 700,000 troops attacking along the line. The fierce resistance of I Corps slowed the pace of the Chinese attack and the Communist assault came to halt on 30 April. It was renewed on 14 May, and met by a UN counteroffensive. The Soviet Union proposed a ceasefire on 23 June, but two months of negotiation brought little progress, and the UN renewed limited attacks. The result was a Communist request for an armistice, and negotiations began at Panmunjom, a village between the lines, on 12 November. Negotiations dragged on through 1952 whilst local fighting persisted. Renewed Communist peace efforts, initiated on 28 March, led to an exchange of prisoners of war, but negotiations stalled when the South Korean leadership refused to accept a divided Korea, provoking further conflict. Negotiations began again on 10 July and by the 27th an Armistice was signed. The de facto border, which has held to this day, was the final battle line.
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