"Oh, hell no! We better stop da communism in its tracks!" Or something like that.
The US fought for South Korea. they were just to help South Korea because North Korea is invading them.
They were sent into South Korea at the request of the United Nations to protect Sout Korea and to eject the invading North Koreans.
the United Nations
No...
The United Nations authorized a U.S. led coalition to intervene in the Korean conflict in 1950. The troops were meant to aid South Korea against invading North Korea, who was backed by China. The war would last for three years.
The Democratic People's Republic of Korea (DPRK, or North Korea) has been a member of the United Nations since 1991-09-17.
Korea itself isn't but both states that make up the Korean Peninsula are. These are the Republic of Korea (RoK - South Korea) and the Democratic People's Republic of Korea (DPRK - North Korea).
The United Nations.
North Korean invading forces
North Korea was supported by the Chinese and South Korea was supported by the United Nations (predominantly the US).
The US was part of a United Nations force fighting North Korea and its allies.
North Korea