The 38th Parallel
North and South Korea were split based off of who was communistic and who wasn't. South Korea remains anti-communistic as North Korea remains communistic. Both these sections had many battles and they tried to restrain the spread of communism.
War in Korea
The North Koreans attacked the South Koreans and the Soviet Union were betting whether or not the US would supply the South Koreans. Instead, they eventually ended up helping the South Kroeans during the war. They fought along the 38th parallel making a tense situation between both parts of the nation.
Fighting with the Chinese
China comes into the picture and starts fighting with Korea. They basically ambush everything and end up capturing the capital of Korea, Seoul. This causes more anger between the Koreans and Chinese.
Communistic then and Communistic today. The separation given off by the 38th parallel and the influence from the Soviet Union cause them to be Communistic today.
South Korea was always non-Communistic. Even though at one point Soviet Union supplied them, they were uninfluenced and thankfully the 38th parallel kept them like that over the course of several years.