Korea was forced to split upon Japan's surrender per the Potsdam conference agreement: http://en.wikipedia.org/wiki/Korean_War
38th parallel divided Korea before the Korean war, the 38th parallel was built before the war in 1945
Korea was individual country. No one colonized or ruled Korea before Japan.
Japan controlled Korea from 1910 to the end of world war 2. After the war, the Allies divided Korea at the 38th parallel. In short: Cutting up Korea.
North of the 38th, the Soviet Union accepted the surrender of Japanese forces, leading to the creation of a Communist North Korea.
For purposes of accepting the Japanese surrender and providing postwar security in Korea and to temporarily divide Korea into northern and sourthern parts.
The USSR and the US
No. Japan occupied all of Korea from before WW1 until 1945