This claim is false: Japan had no interest in attacking the USSR at this period of time.

To understand why this is the case, we need to look at Japan’s strategic situation during 1941. First off, Japan was in an entrenched war in China. This war was dragging a large amount of Japanese resources into it which made ending it grow increasingly harder as time went on. Because of Japanese aggression, the United States froze Japanese assets in the US, most importantly oil which the Japanese needed. This crippled the Japanese economy.

Japan as a result needed to take a location that offered a large amount of resources in which they could harvest to finish their war in China and provide security in any future wars. This area soon became known as the “southern resource area” and featured the Philippines, Malaysia and the surrounding islands.

However some did favor an altercation with the USSR. But this had more disadvantages over advantages. First off, this meant that Japan would be fighting another major power, whom last time they fought beat the Japanese in Manchuria. Secondly, it would result in war with the UK. They were okay with this but a war with China, the commonwealth ( most notably, India ), and the USSR meant a huge frontline they’d need to fight on. Remember they were already in a war with China, and no resources would be of great help from what territory they could take in Siberia. It also seemed likely the Us would join the war if the Japanese acted aggressively in the pacific. They thus would be fighting 4 major powers. This plan though did have supporters, most importantly was the Japanese army. The Navy were against this.

The other plan was favored a strike against the US fleet to cripple it. Then the Japanese could take the southern resource area as their main rival would be out of the fight. This could then support their armies in China. The Japanese knew the US would retaliate but thought westerners to be weaker than them and didn’t think the US had stomachs for a war with them. They wouldn’t need to worry about the Soviets because they had a non aggression pact with them. The Navy was for this as it made more sense to use the Japanese navy instead of only relying on the army. The plan to attack Russia had no use of the Japanese navy ( at least not on any large scale ) but if the Japanese attacked the US, Japan, one of the worlds leading naval powers could thus use one of its largest resources. But one of the best parts about this plan is that the Japanese and USSR had a non aggression pact, making war with them unlikely.

So in summery. The Japanese had more to gain from a war with the USA/UK/China and other allies than a war with the USSR/UK/ China, other allies and maybe the US. So they went with the plan to attack the US. If Moscow fell would they have joined? Most likely not because they had troops in place for their war with the western allies and would need to divert more troops to Manchuria. Not to mention the fact that during the war, the Japanese refused to sink any ships heading to the USSR from America, out of fear of angering the USSR.

My sources are “ww2 at sea” 2019, by Craig Symonds.

Also “Inferno” 2011 by Max Hastings.