Forward to Pusan!
In 1950, Soviet Unionball developed his very own atomic bomb from spies working for USAball and UKball. When this was revealed, the world that if a major war broke out between Communists and Capitalists, it would destroy the entire world. Tensions also were rising elsewhere, most notably in Asia. Republic of Chinaball was defeated by communists in the Chinese Civil War and fled to Taiwan. Even then, the newly communist Chinaball was excluded from UNball, and in his place was the pro-USA, capitalist Taiwanball, and soon, all of the world's eyes turned to a specific peninsula.
After World War II, Japanball was defeated and was forced to surrender, and Soviet Unionball and USAball each had a chunk of the Korean Peninsula. The division was meant to be only temporary, but things went sour quickly. In the north, Soviet Unionball created North Koreaball, and in the south, USAball (with black magic) created South Koreaball. They were divided along the 38th Parallel. At home, both Koreaballs dreamed of defeating the other and unifying the country under their control. And so, now that Soviet Unionball had his very own nuke, he felt more confident than usual, and so he allowed North Koreaball to invade. And so he did.
North Koreaball steamrolled through South Koreaball's clay, taking Seoulball really quickly, and replacing worst Korea with best Korea. However, back at home, USAball and UNball were freaking out. Just years before, USAball promised to contain the spread of Communism, now if Korea fell to communism, he believed, other countryballs would soon fall to communism as well. USAball pushed for a resolution in UNball, and to help South Koreaball win the war. They still held Busanball, after all. Maybe not all hope was lost. And so it was.
Because of USAball's push for resolutions through UNball, they agreed and quickly created an emergency force made up of troops from 14 different countries to aid South Koreaball. At Busanball, South Koreaball was still trying to hold out, but UNball came to its rescue. They made landings at Incheonball, near Seoulball, and trapped North Koreanballs at the South Korean peninsula. South Koreaball and UNball pushed North Koreaball back up, replacing best Korea with the worst Korea. And they kept going until they captured Pyongyangball. And we're so close to defeating North Koreaball, when Chinaball got paranoid. Chinaball got paranoid even as UNball was pushing up into Seoul. He did not want to lose an ally in Asia, not to mention, if UNball defeated North Koreaball, they might just keep going up into Manchuriaball. And so, Chinaball made his decision. He would help North Koreaball liberate the motherland. Just as UNball was at the Chinese border, about a million Chinese troops crossed the Yalu river into North Koreaball. This new offensive took everyone by surprise (again), and Chinaball just as easily steamrolled through the UNball forces, and North Koreaball, with the help of his communist friend, recaptured Seoulball in 1950. However, UNball's counterattack was powerful and effective and they pushed Chinaball and North Koreaball back over the 38th parallel, recapturing Seoulball and parts of North Koreaball's clay. After this initial success, however, the war went nowhere. No progress was made, no defeat of best Korea (thankfully), no glorious push to liberate the motherland, the war just stopped.
The fighting ended on 27 July 1953, when the armistice was signed. This created a De-militarized zone in Korea, monitored by troops from both sides. This has created a frozen conflict, still continuing to this day. See below.
After the armistice
Since the armistice, there have been numerous incursions and acts of aggression by North Korea. In 1976, the ax murder incident was widely publicized. Since 1974, four incursion tunnels leading to Seoul have been uncovered. In 2010, a North Korean submarine torpedoed and sank the South Korean corvette ROKS Cheonan, resulting in the deaths of 46 sailors. Again in 2010, North Korea fired artillery shells on Yeonpyeong island, killing two military personnel and two civilians.
After a new wave of UN sanctions, on 11 March 2013, North Korea claimed that it had invalidated the 1953 armistice. On 13 March 2013, North Korea confirmed it ended the 1953 Armistice and declared North Korea "is not restrained by the North-South declaration on non-aggression". On 30 March 2013, North Korea stated that it had entered a "state of war" with South Korea and declared that "The long-standing situation of the Korean peninsula being neither at peace nor at war is finally over". Speaking on 4 April 2013, the U.S. Secretary of Defense, Chuck Hagel, informed the press that Pyongyang had "formally informed" the Pentagon that it had "ratified" the potential usage of a nuclear weapon against South Korea, Japan and the United States of America, including Guam and Hawaii. Hagel also stated that the United States would deploy the Terminal High Altitude Area Defense anti-ballistic missile system to Guam, due to a credible and realistic nuclear threat from North Korea.