<P> Since the Sino - Soviet Treaty of Friendship resulted in the Soviet Union losing its naval bases in Port Arthur and Dalian , the issue that Stalin had to face was to find a new ice - free port as a Soviet naval base . This issue motivated Stalin 's support for allowing Kim Il - Sung to launch the Korean War . However , both Kim Il - Sung and Stalin did not consider that the United States would intervene into that war immediately , if at all . Kim Il - Sung could not sustain the attack against the US Army . When Kim Il - Sung required military assistance from the Soviet Union and China , Mao agreed to send Chinese troops , but asked the Soviet Air Force to provide air cover . As the two leaders distrusted each other , Stalin agreed with sending Chinese troops to Korea , but refused to provide air cover . Since without the air cover from the Soviet Union , Mao once considered that China did not send troops into Korea , and Stalin at one time decided to give up the Korea Peninsula . After much cogitation , Mao solely sent Chinese troops into Korea on 19 October 1950 under an extremely hard Chinese economic and military situation . This activity ultimately changed the Sino - Soviet relationship . After 12 days of Chinese troops entering the war , Stalin allowed the Soviet Air Force to provide air cover , and supported more aid to China . Mao sending Chinese troops to take part in the Korean War was followed by large - scale economic and military cooperation between China and the Soviet Union , and the friendly relationship of the two countries changed from titular to virtual . In one less known example of the Sino - Soviet military cooperation , in April -- June 1952 a group of Soviet Tupolev Tu - 4 aircraft were based in Beijing to perform reconnaissance missions on American fusion bomb tests in the Pacific . </P>
when did relations between china and the soviet union begin to improve
Chinese troops to take part in the Korean War was followed by large - scale economic and military cooperation between China and the Soviet Union