On 25 June 1950, North Korea launched a surprise attack on South Korea, commencing the Korean War and aiming to unify the country by forces. On 7 July, when the UN adopted a resolution calling for all possible means to aid South Korea, President Truman announced he was sending in US forces to stop the Communists and expanding his Cold War containment to East Asia. The Korean War became a conflict between China and the United States. To drive UNF out of Korea, Mao sent in 33 divisions, which was only the beginning of Chinese involvement. In April 1951, CPVF launched its spring offensive against UN troops, which put up a strong defence. After the Chinese failure in the battle, the war settled into a stalemate and a more conventional pattern of trench warfare along the 38th Parallel. The Korean stalemate became the longest positional warfare in world military history. This military impasse, from June 1951 to July 1953, has become the most forgotten phase of the ‘forgotten war’. About 45 per cent of all US casualties occurred after truce talks began in July 1951. By 1952, Chinese forces in Korea had grown to a record high of 1.45 million. Realising the huge gap between Chinese objectives and means, Mao became willing to accept a settlement without total victory.