 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.