As agreed with the Allies at the Tehran Conference in November 1943 and the Yalta Conference in February 1945 , the Soviet Union entered World War II 's Pacific Theater within three months of the end of the war in Europe . The invasion began on 9 August 1945 , exactly three months after the German surrender on May 8 ( 9 May , 0:43 Moscow time ) . Although the commencement of the invasion fell between the American atomic bombing of Hiroshima , on 6 August , and only hours before the Nagasaki bombing on 9 August , the timing of the invasion had been planned well in advance and was determined by the timing of the agreements at Tehran and Yalta , the long-term buildup of Soviet forces in the Far East since Tehran , and the date of the German surrender some three months earlier ; on August 3 , Marshal Vasilevsky reported to Premier Joseph Stalin that , if necessary , he could attack on the morning of 5 August . At 11 pm Trans-Baikal ( UTC +10 ) time on 8 August 1945 , Soviet foreign minister Vyacheslav Molotov informed Japanese ambassador Naotake Satō that the Soviet Union had declared war on Japan , and that from 9 August the Soviet government would consider itself to be at war with Japan
Yalta Conference
Yalta Conference
Russia involvement in regime change