He promised to give Soviet Union(USSR) the eastern part of Poland and Japan would have to return the territories ir had seized from the USSR.
In the Yalta Conference,where Britain,USA and USSR participated to make decision on post-war settlements,USA president Roosevelt promised USSR Premier Stalin that the eastern part of Poland would be given to USSR and Japan would have to return the territories it had seized from the USSR in exchange for the Soviet Union joining the Pacific War against Japan.
In addition, Germany would be devided into 4 zones, seperately occupied by USA,Britain,France and USSR.
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