Why did the war break out in Korea?

the UN is supposed to enforce/arbitrate world order.........therefore when NORTH KOREAN troops envaded South Korea, the UN (and USA) stepped in to assist the South Korean government to stem the invasion tide from North Korea. The many countries that fought against north Korea were members of the UN
A similar reason to the war in Vietnam. The war against communism. The US believed in the domino effect, in that if one country fell influence to communism, the neighboring countries would fall as well. Therefore, South Korea was supported by the US to promote capitalism, whilst North Korea was supported by China to promote communism.