Sakhalin .
Russia
Sakhalin Oblast
Sakhalin Island.
The island of Sakhalin, known as Karafuto in Japanese, lies in the Sea of Okhotsk northward of the Japanese island of Hokkaido and eastward of Russia. It lies between latitude 42 degrees 50 minutes north and 54 degrees 24 minutes north. The island has a length of 942 kilometers or 585 miles and a maximum width of 160 kilometers or 100 miles. Today Sakhalin has a population of 546,695 of whom 83% are ethnic Russians. A population of approximately 400,000 Japanese inhabitants who used to live on the island were deported in 1945 following the conquest of the southern part of the island by Soviet forces at the end of world war II. Sakhalin remained part of the USSR until its cessation and since 1992 Sakhalin has been a part of the Federation of Russia since.
Japan
Sakhalin.
none
I think the answer you are looking for is the Island of Sakhalin, but I am not 100% sure, as it was not split in 1918, but in 1905 and Russia was not yet Soviet Union neither in 1918 nor in 1905.
The Pacific Ocean.
yes, as many as 13,000
Sakhalin Island.