So the first question you may be thinking about is what in the first place was it that caused the split of Korea?
It began in 1904 with Russia and Japan fighting over Korea. And when Japan finally won Korea became a Japanese Colony until around 1910. Orginally it was America to take over North Korea but the Soviets also showed intrest because both forces realised that Japan controling Korea gave them too much power in the war. So working alongside each other they then went to war with the Japanese. Until Japanese forces in Northern Korea surrendered to the Soviets, while those in Southern Korea surrendered to the Americans and they were then supposed to arrange for nation-wide elections to rejoin Korea in 1948, but neither side trusted the other. The US wanted the entire peninsula to be democratic and capitalist; the Soviets wanted it all to be communist. It wasnt going to work. So to simplify the story North Korea became communist and South became democratic and capalist.More than sixty years and millions of lives later, as a result of the almost accidental division of North and South Korea the 38th parallel remains arguably the most tense border on Earth.