Operation Zet was a secret operation of the Soviet Union to provide military and technical resources to the Republic of China as a part of the Sino-Soviet Non-Aggression Pact.
The operation was aimed at resisting the invasion of the Imperial Japanese Army during World War II. The Soviet Union and The Empire of Japan had a longstanding rivalry that sometimes had resulted in open hostilities.
The operation was carried out in secret to maintain plausible deniability while simultaneously resisting the expansionist efforts of Japan to establish itself as the pre-eminent political force in East Asia. Under the name of Soviet Air Force Volunteers, Soviet troops fought in the defense of Nanjing, Wuhan, Nanchang and Chongqing. Over 250 Soviet volunteer pilots and 885 aircraft were provided to China. The aircraft include Polikarpov I-15, Polikarpov I-16 and Polikarpov I-153.