The correct answer is: "the United States".
The Korean War was fought from 1950 to 1953 between North Korea, supported by China and the Soviet Union, and South Korea, whose principal ally was the United States.
This war was embodied in the Cold War conflict, between the Eastern and Western blocs of countries, leaded by the URSS and the US respectively. The URSS was the main communist power while the US was the most powerful capitalist state, and they were confronted and aimed to impose their form of state organization all over the world.