Stalin uses his formidable forces in 1937

OTL in 1937 the USSR had large numbers of the only all metal fighter with retractable landing gear and SBD twin engine bombers. It also had the huge TB-3 to transport troops long distances, more tanks than the rest of the world combined, etc,
Russia had been humiliated and lost territory and its navy to Japan in the early XIX century. Japan had expanded its armed forces enormously with resources from Taiwan, Korea and Manchuria. Should Japan defeat China, it would have even more resources and unlimited human power for its armed forces, war industry, agriculture, etc, Moreover, Japan was friendly with Poland and Germany, so the USSR could be forced to fight in two very distant fronts simultaneously.
 
Moreover, any major failure in the top leadership in combat could justify execution much better than mock trials. Accordingly, Japan decides to put Tukhachevski in charge of a huge army to invade Manchuria, Sakhalin and Korea, while the small IJA is engaged invading China. The USSR will also supply armament, munitions, etc, to China in order to maximize Japanese casulties and supply Chinese Communists only if they stop fighting Chiang and fight in Japanese territory. to aid the USSR.

The USSR promises Roosevelt to arm Cand assist China and to expel Japan from the continent and liberate China, provided the USSR retains Manchuria, Korea and Sakhakin and the USSR ships tractors, trucks, fuel, grain, beef, etc, to Vladivistok for the Ofensive and provides escort for the convoys. China is also to receive  US aid to bleed the IJA. Without Manchuria, Japan will not be able to expand nor attack other countries. If it does, it can be bombed or invaded from Korea.

Tukhachevski deploys 5 divisions and Mongolian cavalry, 1,000 planes and 1,000 tanks to prepare for the attack, as supplies and reinforcements arrive from the erst and US supply reach Vladivostok. He attacks when the Japanese are busy in Shabghai in Oct, 1937 (experiencing heavy losses thanks to generous Soviet aid and air and armor support) and while a larger force is en route from the west.

  Japanese forces are spread thinly along the extremely long Manchurian border and are out flanked or overrun in several key places. Huge TB-3 land thousands of troops deep in Manchuria. They capture vital RR, RR hubs, roads, crossroads, bridges, munitions and supplies depots, disrupt communications, etc, IJA HQ have to redeploy troops from Shanghai, providing a respite for Chiang's beliegered forces), but they take a while to arrive in Manchuria and Korea, so the red army penetrates deeply during the initial weeks. Soviet I-16 quickly anihilates Japanese biplanes and fixed landing gear monoplanes (A5M, Ki 27, bombers, etc,). SBD also destroy many planes on the ground and aviation fuel tanks, runways, barracks, repair shops, etc, Since the IJA depends on RR transportation in Manchuria, RR disruption ruins Japanese supplies and redeployment. IJA troops must march very long distances, using horses to pull equipment to the front, while the red army can fly troops and light artillery and munitions 1,000 km in a day using TB-3. Accordingly, Tukhachevski out maneuvers the IJA all over Manchuria and Korea. Sakhalin falls in 9 days, without any Japanese escaping. 
The weakened IJA offensive on Shanghai stalls and Chiang's forces are able to counter attack with Soviet air and armor support, causing heavy casualties, which the Japanese cannot replace. Eventually, the IJA must withdraw from Shanghai, but the troops cannot be redeployed to Manchuria in time, so they return to Japan. Isolated IJA troops in Manchuria and Korea run out of food and munitions and almost all surrender by Feb, 1938. Japan has lost its source fo soy, grain, opium, coal, metal ores, lumber, and much of its heavy industry.