On September 1 , 1945 , or one day before the surrender documents of World War II were signed on September 2 , 1945 , in accordance with decisions made at the Yalta Conference , the Soviet Union acquired the Kuril Islands . This occurred after the Soviet Union renounced the Soviet – Japanese Neutrality Pact in April 1945 and declared war on Japan on August 9 , 1945 ( formally , the pact itself remained in effect until April 13 , 1946 ) . Although Japan agreed after deliberations to cede its claims on the entire island chain including the '' Northern Territories '' as part of the Treaty of San Francisco in 1951 , the Japanese government has claimed since the 1960s that the southern islands were not part of the ceded Kuril Islands
Yalta Conference
Yalta Conference
Kunashir Island