Hitler declared war on the US in the hopes the Japanese would help him out against the Soviets.
The Japs weren’t stupid. They had their hands full without having to deal with the Russians.
Hitler wanted the Japanese to invade Siberia.
But the Japanese had already had a couple of go-rounds with Soviets at Lake Khasan and Khalkhin Gol and did not want any more of that.
That freed up the Soviet forces in Siberia to fight the Germans and probably changed the outcome of WWII.