 In September 1937, they signed the Sino-Soviet Non-Aggression Pact and approved Operation Zet, the formation of a secret Soviet volunteer air force, in which Soviet technicians upgraded and ran some of China's transportation systems.Bombers, fighters, supplies and advisors arrived, including Soviet general Vasily Chuikov, future victor in the Battle of Stalingrad.Prior to the Western Allies, the Soviets provided the most foreign aid to China