From Nanjing 1937: Battle for a Doomed City, by Peter Harmsen (Casemate, 2015), Kindle Loc. 546-565:
The Soviets had good reason to be circumspect. The alliance with Chiang was not based on ideology but was born out of a convergence of strategic interests. China was looking for a new source of overseas assistance, as Germany, its main foreign backer up until then, had shown itself to be an unreliable partner, gradually moving closer to Japan. The Soviet Union, on the other hand, saw a cynical benefit in supporting China’s war, as it would keep Japan too preoccupied to threaten its eastern borders.
This marriage of convenience had manifested itself in a Sino-Soviet non-aggression pact, signed in August 1937. The Chinese had wasted no time, and had sent a wish list of 350 planes—and pilots—to Moscow even before the agreement was inked. At the end of the day, the Soviet leaders opted for less ambitious aid, agreeing to 200 planes, in return for Chinese delivery of minerals essential for war production, such as wolfram and tungsten.
The Sino-Soviet friendship received support from a very unlikely source—British politician Winston S. Churchill. The Soviet envoy to the United Kingdom described how in a meeting Churchill “greatly praised our tactics in the Far East: maintenance of neutrality and simultaneous aid to China in weaponry.” This was for the best, he thought, since a more open backing of China would raise the specter of an expansionist Soviet Union, a lingering fear among many powers, thus making the situation easier for Japan and complicating the establishment of “a grand alliance” directed against Germany, Japan and other regimes. Intriguingly, even at this early stage, Churchill saw such an alliance as “the only means of saving mankind.”
Indirect aid didn’t mean an absence of risk. Russians would still be put in harm’s way and Rytov knew that. Later on the same day that he was told he would be going to China, he met up with another member of the coming mission, Pavel Vasilievich Rychagov, who had recently returned from a successful tour as a fighter pilot in the Spanish Civil War and had been awarded the Lenin Order twice for his service there. Together, they were briefed by Yakov Vladimirovich Smushkevich, the deputy commander of the Soviet Air Force. “The Japanese armed forces are technically superior to the Chinese,” said Smushkevich, who was also a veteran of the Spanish conflict. “The Chinese Air Force is a particular concern. Soviet pilots who have rushed to China’s aid are currently in Nanjing. They are fighting valiantly.”