When General Douglas MacArthur had received news that the People's Liberation Army had engaged (and in some cases annihilated) American forces, he acted quickly. Previously, a number of nuclear weapons had been transfered to Korea specifically in case of a Chinese attack, but President Truman had not authorised their use. MacArthur's fame and contacts, however, allowed him to convince local Air Force commanders to use these weapons to "save democracy". On April 2nd, 1951, the city of Sinuiju in North Korea was atomised by a single Mark 4 fission bomb, dropped from an American B-29. The blast also destroyed and irradiated the Chinese city of Dandong. Though Sinuiju had enormous value as a logistics hub for the PLA, the use of only a single bomb was akin to an escalated threat. In response, the Soviet Union announced a nuclear exchange program with the PRC, quickly transfering a number of nuclear weapons for "national defence". Between these was the rushed, experimental RDS-4 prototype Teller-Ullam hydrogen bomb.
It was this device which was detonated in the Yellow Sea just 80km from the South Korean city of Mokpo, with a yield of 2.6 megatons, far greater than any American bombs at the time. MacArthur had been relieved and court-marshaled just a day after his rogue bombing, and as the severity of the Chinese ocean test reached Truman, he quickly contacted Nikita Khrushchev to reach an understandment. A public agreement was created, in which no countries involved in the Korean War (including the USSR) would use nuclear weapons, or other weapons of mass destruction. The grim memory of the destruction of Minsk and Konigsberg in 1945 by atomic weapons was instrumental; both countries' leaders had seen the effects of these weapons first-hand.
The war became one of attrition, with neither side able to push the other. In 1953 renowed ex-Supreme Commander of the Allied Forces, Dwight D. Eisenhower, became President of the United States. More militarily experienced than Truman and much more sane than his competitor in the presidential race, MacArthur, he ignored pleas from his advisors to nuke Chinese nuclear storage facilities, which the US Air Force had spied on for more than a year. Despite heavy opposition from both South Korean and American leadership, he successfully pushed for an armistice, which was signed by all parties on July 27th, 1953.
Peace had been restored. However, by the mid-1960s, the Korean border remains one of the most militarised zones in the planet, and one of the prime "hot-spots" for a future World War 3. This war's most important result was bringing Mao Zedong's PRC and the Soviet Union into a de facto alliance on all military and economic matters, which would culminate in the formation of the Canton Protocol in 1968.