declare war on Japan once the Nazis were defeated is your answer.
In the years before the declaration of war by the USSR on Japan (following the two nuclear attacks on Hiroshima & Nagasaki), the USSR had tried to keep a good relationship with Japan to avoid fighting a two-front war (which would be extremely hard considering the great distance in between European Russia and Asia Russia. However, if USSR was able to defeat one and then the other, it would be easier.
This led to the decision to help the Allies take pressure off the landing of the French beaches (Sword, Juno, Omaha, Utah, etc), and was able to 'tie up' large amounts of German troops in the east.
In return, the USSR declared war on Japan following the bombing of Hiroshima & Nagasaki (as said above), and the declaration of war on Japan, leading to Japan's surrender.
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