The Korean War made the Cold War really hot. Since Korea was partitioned as North and South (1945), each side sought to win legitimacy by unifying the country. The President of North Korea Kim II (People’s Republic of Korea) decided to act before his southern rival, Syngman Rhee (the Republic of Korea), got the chance. He launched an attack in June 1950, with the tacit support of Stalin. Both Kim and Stalin did not expect the US to intervene. The war lasted for three years. The human cost was enormous. But the Korean people gained nothing.
The capitalist countries led by the U.S. were politically designated as the First World, while the communist states led by the Soviet Union came to be known as the Second world. States outside these two were called Third World.