Korea is located on the Korean peninsula, bordered by land predominantly by China in the northwest, as well as a small border with Russia in the northeast. Japan is situated across the Korea Strait to the east. It has been heavily influenced by Chinese and Japanese culture, especially during its Japanese occupation from the early 1900s to the end of World War II in 1945.
During its occupation, Korean citizens were forced to take Japanese surnames and were forbidden from keeping any relics of Korea's past. It's during this time that the more widely known Korean martial arts were forcefully eradicated by the Japanese government, and the Japanese martial arts system was put in place.
After Japan's defeat in World War II, the USSR & the United States expressed interest in disarming the Japanese troops in that region. The USSR disarmed the Japanese troops stationed in the Northern part of the peninsula, and the US took the southern part. Due to differences in ideology between the USSR & the US, two different sets of governments were established. Korea was divided into 2 distinct countries on the 38th parallel: the communist influenced North Korea and the democratic and more Western-friendly South Korea.
With no more restrictions placed on Korean arts by their occupiers, Korea began to re-establish what they had lost. Taekwondo and Hapkido eventually became the country's national sport, but their ranking systems and initial move sets were heavily influenced by Japanese Karate & Judo. Eventually these martial arts established their own identity upon the world.
Several years later, South Korea was invaded by North Korean forces resulting in a long and bloody war. With millions of civilians being slaughtered, and the South Korean government losing ground each day, the United States decided to help the south. With the aide of US forces, South Korea was close to overtaking the entire Korean Peninsula.
With the North Korean/USSR troops now losing the war, China knew that the loss of a major communist territory to democracy would not bode well for communism on a global scale, so they decided to act.
Chinese troops poured into the country and quickly pushed back the South Korean/American coalition troops back towards the originally established border on the 38th parallel.
The two massive armies fought to a brutal stand still with massive losses to both sides, until eventually it was decided to sign a ceasefire agreement. Tension on the 38th parallel still runs high as the war has still never formally ended. North and South Korean (and US) troops are still stationed at the border to ensure neither side makes a hostile move.