At the very end of world war 2, Russia invaded Japaneses possessions in Asia, Manchuria and Korea. Shortly after this japan surrendered. the Russians at this time had made it halfway down the Korean peninsula. in the terms of the agreement, the county of Korea was split, similarly to Germany, with the Soviets controlling the north and the U.S. in the south. each side set up their own puppet rulers, Kim Ill Sung in the north and i can't remember who in the south.
Stalin divided the peninsula as he did most of Europe at the end of the war.
Before this Korea had never been divided, it had been under Japanese control for 40 years.
Roosevelt and Churchill made many mistakes toward the end of the war, especially in concessions to the Soviets.
Because one side was Communist and the other one wasnt!
Japan had control over Korea until the end of WWII when they surrendered. The Korean War (shortly after WWII) began as a dispute between the two dominant political parties in Korea: Democratic People's Republic of Korea and the Republic of Korea. DPRK was supported by China and Russia (communist) and RoK was supported by UN allies. They divided the country at the 38th parallel (demarcation line) and tensions rose until "North Korea" invaded "South Korea" and started all out war. Eventually they came to an armistice, or cease fire, and separated the two countries at the Korean Demilitarized zone (DMZ) -- off limits to both countries. (Its a neat experience to visit the DMZ if you ever go to Korea.) Apparently, within the last year South Korea pulled out of the armistice, but I'm not really sure what that means for the war.