It would be an irrational move for Japan or the Soviet Union. They both have more important opponents. But it’s not ASB. Let’s say some rogue Japanese commander assumes the attack on the US and UK will make war with the Soviets imminent and he attacks. Or the Soviets, either their leadership or a single commander, misinterpret the attacks and assume they are going to be targeted so they strike first. Or Japan’s government believes that the Soviets will attacks them for attacking their ally so they attack. Or someone in Japan believes that with Germany’s declaration of war against America means they are obligated to attack the Soviet Union. Or the Soviets believe the WAllies might not unconditionally support them if they don’t go to war with Japan so they attack. Putting aside the “why” and “how” to an extent, how long does Japan last here? Does Japan fight on even if they are booted out of Asia, provided Germany is still fighting?