Korean War

      The Korean War has been called the "Forgotten War"，it began in the early hours of June 25, 1950, when North Korean troops crossed the 38th parallel and invaded South Korea. South Korean was supported by the United Nations, and North Korea, supported by the People's Republic of China (PRC), with military material aid from the Soviet Union. 
    
    There were several factors that US intervened in the Korean War.One facet of the changing attitude toward Korea and whether to get involved was Japan. The United States wanted to support Japan to be a viable counterweight against the Soviet Union and China, and Korea was seen as integral to that end. Another important part that US intervened was Soviet Union's actions.American government worried that this War would increase to become a global war.With the Soviet Union's agreement that this would not cause an escalation, the United States now could intervene with confidence that other commitments would not be dangerous.
    
    On 20 August 1950, Premier Zhou Enlai informed the United Nations that "Korea is China's neighbor... The Chinese people cannot but be concerned about a solution of the Korean question". China warned that in safeguarding Chinesenational security, they would intervene against the UN Command in Korea. On 8 October 1950, Mao Zedong organized the PLA North East Frontier Force as the Chinese People's Volunteer Army (PVA), who were to fight under the command of Marshal Peng Dehuai. There were totally five Phase Offensives. The two parties in 1953 july 27 in panmunjom signed an armistice agreement.
     
    Although an armistice was signed in 1953 between the United Nations, the US, China and North Korea, South Korea refused to sign, leaving the two Koreas separate to this day.

 Source:
    http://en.wikipedia.org/wiki/Korean_War
  http://www.boston.com/bigpicture/2010/06/remembering_the_korean_war_60.html