Also, because the US fought the Japanese, Japan was reluctant to engage the Soviet Union and fight at two fronts. When Stalin learned that Japan was not going to wage war against it, he relocated some fresh Siberian divisions to the west, which helped defeat the Germans in the Battle for Moscow. Not to diminish the decisive role and huge sacrifices of the Russian soldiers and civilians in defeating the Nazi Germany.
Hitler declared war on the US, with the expectation that the Japs would help him with the Soviets.
The Japs weren’t that stupid.
In 1939 there were a series of battles around Khalkhin Gol, Mongolia, in which Soviet Gen. Zhukov outnumbered Japanese forces more than two-to-one, out-maneuvered & decisively defeated them.
From these battles many Japanese took away the lesson: don't mess with Stalin.
But as late as 1941 the Japanese army still wanted to re-fight and defeat the Soviets.
Had they done so (instead of uselessly attacking Pearl Harbor) they would find a much easier time accumulating adequate forces against Russians who were now fighting Hitler.
The result could force Stalin to make peace while preventing American entry to the war.
Fortunately for us, Japanese were in a hurry, underestimated US response to Pearl Harbor, and overestimated Hitler's prospects without them against Stalin.