The correct answer is Korea.
The Chinese government adopted a foreign policy that actively promoted communists revolutions throughout territories around China.
In October 1950, Chinese troops under the name of Chinese People's Volunteer Army crossed the border to assist North Korean armies and engaged in the Korean War in an offensive manner.
The intervention was mainly the security concern of possible U.S. invasion on Chinese territory.
During the conflict, North Korea was supported by China and the Soviet Union while South Korea was aided by the United States.