Korea was occupied by Imperial Japan and sufferd greatly under them. China went communist and during the end of WWII established and benefactored a communist element in Korea, which tried to take over all of Korea, America went to war to stop this influence and was eventually beaten back due to the Chinese involving themselves in the conflict. A armistice was signed and a line was drawn on the map and North Korea became a hermit state due to the nature of dictatorship and the Koreans already war weary of foreign powers. Technically the war was never ended and we've been in a cease fire. Like Western Germany South Korea benefitted from trade with the outside world while North Korea became dependent on China for food and goods and services. They found that by being combative with the west they could gain food and compesentaion due to South Korea's sunshine policy of enough sun and contentment was applied the umbrella would come down and the rain coat off. Only like any bully or desperate tyrant this only embolden the North Koreans to assassinate or bomb South Korea over the years and usage of rhetoric to get more compensation from the west while China enjoyed America's distraction till they finally they discovered that they enjoyed economic success as well and now North Korea is more a hindrance thanks in part to its insanity and isolationism. China has taken the stance of denying North Korea's elite monetary perks along side the West over the development of nuclear weapons and rogue sales of this technology and now you have someone pissed off about not getting his toys waving the saber around like his dad to get the US and China to back down