日ソ中立条約
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.