Short answers, pulled from the story.
The Sino-Soviet Non-Aggression Pact was signed in Nanjing on the 21st of August 1937. It was registered in the League of Nations Treaty Series on the 8th of September 1937, the same day it came into force.
Operation Zet was a Soviet program launched after the signing of the pact in which the USSR sent aircraft and economic aid to the Chinese national government under Chiang Kai-shek. It was intended to help China resist the Japanese invasion.
The Soviets breached the pact by conducting the Xinjiang War from August to October 1937, the same period in which the pact was being signed. Soviet forces supported puppet Governor Sheng Shicai against Chinese Kuomintang troops, deploying mustard gas bombardment against the 36th Division led by General Ma Hushan.
The Republic of China government concealed the Soviet invasion because publicly acknowledging it would have ended the flow of Soviet military supplies China needed to fight Japan. Officials publicly dismissed reports of Soviet troops as "Japanese propaganda" to preserve the supply relationship.
In July 1942, Chiang Kai-shek ordered Ma Buqing to move 30,000 troops to the Tsaidam marsh in the Qaidam Basin of Qinghai. Chiang named him Reclamation Commissioner so the troop presence would threaten Sheng Shicai's southern flank and pressure the Soviet-backed governor.
The Ili Rebellion broke out in Xinjiang in November 1944 after a Kuomintang Muslim officer named Liu Bin-Di was killed fighting Turkic Uyghur rebels. The Soviets backed the Turkic rebels against the Kuomintang, extending the pattern of Soviet interference in Xinjiang that had continued since the pact was signed in 1937.