The Battle of Khalkhin Gol/Nomonhan was a significant Soviet victory. However, it is not clear whether the Red Army could simply "scale up" its Khalkhin Gol/Nomonhan victory against the whole Kwantung Army in May 1940. Zhukov greatly surprised the Japanese by carrying out a large scale offensive far from a railhead. He won, at least partly, because he could supply his forces for the offensive while the Japanese forces were very short of everything, including water (see "Nomonhan: Japan Against Russia, 1939", by Alvin D. Coox, page 568 and pages 579-580). However, Zhukov used every truck that he could find in the Far East and http://www.militaryhistoryonline.com/20thcentury/articles/nomonhan.aspx mentions that he was sent 1625 trucks from Europe. Thus a quick Soviet victory in 1940 might not occur because only a slightly larger operation than Khalkhin Gol/Nomonhan could be supplied. By August 1945, by contrast, the USSR had received and built a much larger number of trucks to supply its August Storm offensive.If we take our cues as to how a potential Second Russo-Japanese war would shape up in spring/summer of 1940 from the Nomohan incident, to say the least the Japanese are screwed.
Japan would probably make peace with CHina and withdraw to pre-1937 borders, and concentrate all of their might in fending off the Soviets in Manchuria. That doesn't leave any room for a Pearl Harbor, besides, they are going to be looking for allies in the West now that they feel that Germany has betrayed them (by making a pact with the USSR).I tried to start a similar thread https://www.alternatehistory.com/discussion/showthread.php?t=131661 and my main question was whether Germany would have tried to launch a late 1940 Barbarossa rather than starting the Battle of Britain. If Germany does attack the USSR in August 1940, they will certainly not take Moscow that year. However, they will not meet any T-34s and the USSR will not receive many supplies from the Anglo-Americans because the Lend-Lease Act is still far in the future and the routes via Vladivostok and Iran are blocked (it would probably be hard to send convoys to Murmansk with Bismarck in Norway).
If Germany attacks, what happens to UK - Japanese relations? Might Japan move South without attacking the USA? Might they be stupid enough to attack the USA in 1940?