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In the final months of the war the Japanse hoped, and indeed it was their only hope, that the Soviets would help them to mediate peace.

Now according to wiki Stalin had very specific goals in Japan which made him reject said offers, or rather, stall the negotiations to allow him build up for the attack and then reject it:

Security concerns dominated Soviet decisions concerning the Far East.[52] Chief among these was gaining unrestricted access to the Pacific Ocean. The year-round ice-free areas of the Soviet Pacific coastline—Vladivostok in particular—could be blockaded by air and sea from Sakhalin island and the Kurile Islands. Acquiring these territories, thus guaranteeing free access to the Soya Strait, was their primary objective.[53][54] Secondary objectives were leases for the Chinese Eastern Railway, Southern Manchuria Railway, Dairen, and Port Arthur.[55]

In the end Japan would had been no worse accepting this since in the end it still surrendered and had to give up everything anyway.

So let's suppose they see the writing in the wall and a prince or diplomat from Japan tells the Soviets they are willing to listen to any terms they might have to offer in exchange for their mediation to achieve at least a conditional surrender with the US, they manage to find out the Soviet goals in declaring war with Japan and offer Stalin everything he wants without having to fight for it at all in exchange for renewing the neutrality pact and mediation to reach a settlement with the US.

Could Stalin had agreed to this?
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