Why, and with what success, did the UN intervene in Korean War?

In the beginning, the United Nations Temporary Commission on Korea was established in 1945 as a general world peacekeeping organization. This was set up to organize elections and built a democratic government. However, North Korea and the USSR refused to cooperate with the commission. On 25 June 1950 military forces from North Soviet Korea attacked the American South Korea. It seemed as a proxy war between the USSR and US, but in fact this attack was mainly decided …

…General MacArthur, who was an American, was leading the UN forces on this war. Finally to answer the topic questions, the UN was rather successful on this mission. But with the US's control, it had to cross 38th parallel and provoke China to join this war, which were not what they meant. But as for the result, the war pretty much ended where it had started, so we can say that the UN was successful.