On August 8, 1945, Russia denounced its neutrality pact with Japan, and the Red Army began their invasion by crossing the 50th parallel of the Sakhalin island. On September 2, V-J (Victory over Japan) Day, Japan surrendered to America. Russia, having only gone 100 km over the border on that day, aimed for Hokkaido and continued their invasion.

The worst casualties happened when 3 Japanese ships transporting 1,700 refugees from Sakhalin were destroyed by Russian submarines. 600,000 POWs were taken from Manchuria and Sakhalin to work at Siberian labor camps, of which 60,000 never returned to Japan. By the end of Russian attacks, Russia occupied the whole of Sakhalin and Kuril islands. Before the invasions, half of Sakhalin and four Kuril islands were Japanese territory. Since the land below the 50th parallel has been disputed, some maps leave the lower half of Sakhalin colored a neutral grey.

After the collapse of the Soviet Union, Japan had increased hope of having the islands returned from the impoverished nation. However, in the late 90's Sakhalin struck oil, rendering the possibility of land return unlikely.

A complete history can be found at: http://karafuto.com/timetab.html

P.S.: This was a nodeshell rescue!