Why did the UN get involved in the Korean War?

I believe the answer to this question is to stop the spread of Communism. If South Korea was taken by China and North Korea, the "Dominoes" might start falling and other countries might want to become Communist. Great Britain, the USA, Australia, and other counties lend their support to stop this communist spread.

b. martin luther king jr. wrote and spoke "i have a dream"

explanation:

i know this 100 percent is correct   b. is the best one. < 3

no every country is held to the same degree as others just some member countries such as the usa and germany are more economically sound so the need to show there support to all others