After Korea was liberated from Japanese control, the USSR and the US agreed to divide the Korea by the 38th parallel. This resulted in two Koreas. North Korea, backed by the USSR and is communist, and South Korea, backed by US and allies.$^7$
Just five years after this division, the North Korean leader, Kim Il Sung, decided to attempt to unify Korea under his rule. In June of 1950, he launched his surprise invasion of South Korea.$^7$
The US rallied the UNSC and an US led UN coalition was deployed to South Korea. By August of the same year, North Korea occupied almost the entire peninsula, apart from a small US defended perimeter in the south east.$^7$
However, under the command of General Douglas MacArthur, US launched several bold counter attacks. Due to his well planned and thought out attacks, US was able to push back the North Koreans back to the border at the 38th parallel.$^7$
The US approved the decision to push forward across the 38th parallel. In November of 1950 US approached the Korean-Chinese border. The PRC feared that the US might continue and invade China. PRC sent tens of thousands of troops into North Korea and, by 1951, pushed the US and UN troops almost back to the 38th parallel.$^7$
The next two years of fighting did not see any change in the border. In 1953, an armistice was signed. A two and half mile wide DMZ separating North and South Korea was established, which still exists today.$^7$