The division of Korea between North and South Korea happened because of the Allied victory in World War II in 1945. After Japan surrendered, the Allies made Japan give up all the colonies they took through military force, even those before WWII started. Even though Korea was a colony of Japan before WWII, Japan still had to give it up because it used military force to take it over. So after 35 years, Korea no longer became a colony of the Empire of Japan. However, on the same day Japan surrendered, the Soviet Union invaded Korea, after having occupied Northeast China.