Siberian Intervention
Japanese forces invaded and occupied Sakhalin in the closing stages of the Russo-Japanese War. In accordance with the Treaty of Portsmouth of 1905, the southern part of the island below the 50th parallel north reverted to Japan, while Russia retained the northern three-fifths. In 1920, during the Siberian Intervention, Japan again occupied the northern part of the island, returning it to the Soviet Union in 1925.