On April 5 , the Soviet Union gave the required 12 months ' notice that it would not renew the five-year Soviet – Japanese Neutrality Pact ( which had been signed in 1941 following the Nomonhan Incident ) . Unknown to the Japanese , at the Tehran Conference in November – December 1943 , it had been agreed that the Soviet Union would enter the war against Japan once Germany was defeated . At the Yalta Conference in February 1945 , the United States had made substantial concessions to the Soviets to secure a promise that they would declare war on Japan within three months of the surrender of Germany . Although the five-year Neutrality Pact did not expire until April 5 , 1946 , the announcement caused the Japanese great concern , because Japan had amassed its forces in the South to repel the inevitable US attack , thus leaving its Northern islands vulnerable to Soviet invasion . Soviet Foreign Minister Vyacheslav Molotov , in Moscow , and Yakov Malik , Soviet ambassador in Tokyo , went to great lengths to assure the Japanese that " the period of the Pact 's validity has not ended "
Yalta Conference
Yalta Conference
Surrender of Japan