On 26 November 1950, a few months after the beginning of the Korean War, the newly formed People's Republic of China intervened in favour of the North Korean forces, in defence of the Communists.
Mao Tse-Tung acted with a massive sending of soldiers who, in a few weeks, repelled the Americans to their starting positions. Let us remember that this war was part of the confrontation between the communist and capitalist blocs and, therefore, was one of the most dramatic proofs of the Cold War between the United States and the USSR.
Korea had been divided into two occupation zones delimited by the 38th parallel following World War II: one under the pro-Soviet regime of Kim Il Sung (father of the current North Korean leader Kim Jong-Un) and the other under a nationalist government supported by the Americans.
The objective of reunifying the two Koreas was never achieved. Both regimes claimed sovereignty over the national territory, so growing frictions were born that led to war, with the invasion of North Korean forces in June 1950.
Later, US intervention was immediate as they saw this invasion as a wake-up call for communist expansion into Southeast Asia. How did the conflict end? In 1951, US President Truman opened negotiations that ended two years later, restoring the previous situation with the border on the 38th parallel.