Ok so the consensus seems to be that Japan would have made a very small contribution to the Nazis if they had attacked the USSR a few weeks after Operation Barbarossa started. They would have easily taken the ports, but due to weak armor, bad supply systems and railway problems, their ground armies could not have advanced very far. And at most would have diverted supplies worth a handful of divisions from the Eastern Front. Probably not enough to tip the tide in Germany's favor. So, after taking Manchukuo in 1931, the Japanese are in a position of relative stability with zero enemies. It's only until 1937 that Japan invades China and devotes enormous resources to defeating her. Let's say in 1936, when the Anti-Comintern Pact was being signed (a very appropriate time), the Germans convince the Japanese to hold off their invasion of China in 1937 with whatever means necessary and get them to prepare for war with the Soviets, promising them enormous rewards and resources. The key thing Germany would have to secretly tell Japan is to not attack until Germany does (this means no border incidents that cause the USSR to deploy enormous reinforcement to the Far East). What would the Japanese have to do in 5 years to get themselves ready to fight the USSR? Clearly, they would need to invest heavily in modern armor, but this could be achieved if they sacrificed their navy for armored divisions. Having a powerful Navy doesn't really matter anyway since the Russian navy in the far east was almost non-existent. In 1936, the Japanese could have been given German Panzer 3 tank designs (which were around in 1936) and the Japanese could begin forming Panzer divisions. (The Germans were already giving the Japanese expertise and technical help on aircraft design, they could have done the same with tanks.) The Japanese could also have been instructed by Guderian and other blitzkrieg masters in the intervening years between 1936 and the invasion of Poland in 1939 so that they knew how to fight effectively. Then after Germany quickly defeats France, Operation Barbarossa begins with a massive Japanese thrust in the Far East. Because the Japanese are not involved with China, the entire Japanese army devoted against Russia would have been equivalent to 51 divisions or 1.7 million men - almost 2/3 of the Nazi forces on the Eastern Front which were pitted against some 18 Soviet divisions. Something on this scale would have made a HUGE difference in the war. The Japanese would have seized enormous amounts of territory in 1941 because they would have armor and tactics while causing Stalin to send at least 30 divisions to the East. They would have caused a complete route of the Soviet Far Eastern forces in 1941, and would have bogged down the Soviets in 1942. If Hitler took Moscow in 1941 on top of all this, I think the pressure would have been too much for the Soviets to handle.