Look at the list of the United Nations; the UN fought there.
United Nations: Republic of KoreaAustraliaBelgiumCanadaColombiaEthiopiaFranceGreeceLuxembourgNetherlandsNew ZealandPhilippinesSouth AfricaThailandTurkeyUnited KingdomUnited States Communist: Democratic People's Republic of KoreaPeople's Republic of ChinaSoviet Union
South Korea, also known as the Republic of Korea.
China and the Soviet Union were the primary nations that provided aid to North Korea during the Korean War. China sent troops to support North Korea, while the Soviet Union provided military equipment, financial assistance, and diplomatic support.
Red China, mostly, but the Soviets gave them some aid, as well.
The United Nations Command (UNC) is the unified command structure for the multinational military forces supporting the Republic of Korea (South Korea or ROK) during and after the Korean War.
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).
There are two nations on the Korean peninsula because after the Korean War, Korea was divided into two different nations, namely North Korea and South Korea.
One, Korean Republic only has Korean Language widely spoken in their own country. That does not include dialects spoken.
ROK=Republic of South Korea (still exists today).
The Korean war was started when North Korean troops crossed over the 38th parallel and invaded the Democratic People's Republic of Korea to the west and the Republic of Korea to the south.
Against the North Koreans and The Peoples Republic of China were forces made up from most of the members of the United Nations.
The United States felt that it was in their best interest to preserve South Korea as an independent nation, and entered in a treaty where the US agreed that it would aid and support South Korea. This was also an action endorsed by the United Nations- during the Korean War, several nations besides the US provided military support to South Korea.