Attacking Russia did not immediately offer Japan oil, which was what they needed to keep their war machine going. Additionally, Stalin kept substantial forces in the East even when things were at their most desperate in the West. The Red Army probably could have inflicted severe defeats upon the IJA, possibly expelling them from mainland China. However, Soviet gains in the East would probably be limited for lack of forces to exploit the situation. The Chinese resistance would have done the rest of the work once the Japanese run out of all their necessary resources however.
So really, a Japanese attack on the SU only makes it so Red forces who weren't doing anything OTL have something to do. But this course of action removes the possibility of Japan being the catalyst for USA entrance to WWII. That *might* give the Nazis a little bit of an edge to win in Russia, or at least fight to a truce.
And I'm going to start wanking things for the Axis now but...
If Japan enters the war against Russia, Churchill might feel obligated to declare war on Japan, the same way as they did against Finland. Given the Royal Navy's state in mid 1941, it would be a one-sided curbstomp, and the Japanese might be able to seize the Southern Resource Area at the same time they are being crushed in Siberia. There's probably not enough oil for both operations to go off completely, but I think there's a slim, slim chance that the Japanese could pull it off. The Royal Navy was barely able to hold their own against the Italians during this time period, let alone the entire IJN. It would be a walkover for the Japanese,
if they have enough oil for a one-way sailing ticket to the DEI.
FDR might want to defend the Allies, but Congress probably won't declare war to defend European colonies. Given the philosophy of the Japanese government, they might try to attack the Americans as well because the Philippines cover their supply lines. At which point the Axis is screwed because the USA is involved months earlier and Japan is getting thrashed by the Red Army. But I digress, back to the Axis wank.
Given:
1) Japan declares war on the USSR same time as Nazis;
2) Britain declares war on Japan the same time as they do with Finland;
3) Japan can scrape the bottom of the barrel to seize Singapore, Malaysia, and the Dutch East Indies WHILE invading Siberia;
4) The USA does not rise to defend the Allies;
5) The Japanese don't actually
provoke the USA:
They Axis will have a pretty good shot at winning the war. Controlling the DEI without going to war with the USA is Japan's golden ticket. From there on, it can wage war in China to its heart's content and destroy the British presence in the Indian Ocean. If losses for the British in the Far East are bad enough, you might start to see butterflies like the fall of Malta, which is one of those almost-breaking points for the European theater.
I'm sure someone else can weigh in on the logistics of a Japanese lunge at the DEI while invading Siberia.
