Answer:
The Korean peninsula had been occupied by the USSR and the USA after since 1945, taking the 38th parallel as the dividing line. The outbreak of the Cold War resulted in the partition of this country into two states: North Korea, where in 1948 a "people's republic" was established, that is, a communist dictatorship led by Kim Il Sung; and South Korea, where that same year Syngman Rhee established a strong pro-American dictatorship.
The triumph of the communist revolution in China on October 1, 1949 completely altered the geostrategic balance of Asia. Stalin, who had suffered serious setbacks in Europe (failure of the Berlin blockade or the Yugoslav schism), could not resist the temptation to recover ground in Asia and agreed to a North Korean attack on South Korea.
Thus, on June 25, 1950, Kim Il Sung's troops crossed the 38th parallel and advanced triumphantly to the south. The South Korean regime could barely maintain a small territory around Pusan.
The American reaction, to Stalin's surprise, was immediate. Washington called for the convening of the UN Security Council and won a mandate to lead an army to face the North Korean aggression. The absence of the Soviet delegate, who had refused to attend the meetings of the Council in protest at the American refusal to accept the People's China in him, led to this resolution.
The multinational troops of the UN, in fact the North American army under the command of General MacArthur, quickly recovered the lost ground and on October 19 they took Pyongyang, the capital of North Korea.
Three days earlier, on October 16, Chinese troops with massive Soviet military support penetrated into Korea by recapturing the US Army. On January 4, 1951, the communist troops retook Seoul.
At that time, MacArthur proposed the atomic bombing of northern China. Both President Truman and the majority of Congress reacted in alarm to a reaction that could lead to a nuclear confrontation with the USSR. In an increasingly open confrontation, Truman dismissed MacArthur amid the protests of the Republican right and replaced him with General Ridgway.
The USSR for its part expressed its intention not to intervene in the conflict and its desire that two different systems coexist in the peninsula. The "military tie" led to the opening of negotiations that would conclude in July 1953, shortly after Stalin's death, with the signing of the Armistice in Panmunjong. In it a new line of demarcation was agreed that meanders around the 38th parallel.
The survival of the North Korean communist regime has meant that this "Cold War frontier" has been the only one that has survived until the 21st century.
