...was a war between the Republic of Korea (South Korea), supported by the United Nations, and the Democratic People's Republic of Korea (North Korea), supported by China and the Soviet Union. It was primarily the result of the political division of Korea by an agreement of the victorious Allies at the conclusion of World War II. The Korean Peninsula was ruled by the Empire of Japan from 1910 until the end of World War II. Following Japan’s surrender in September 1945, American administrators divided the peninsula along the 38th parallel, with U.S. military forces occupying the southern half and Soviet military forces occupying the northern half. The failure to hold free elections throughout the Korean Peninsula in 1948 deepened the division between the two sides: by the end of the decade, a communist dictator enjoyed the support of the Soviets in the north, while in the south an anti-communist dictator was supported by the US government. Border skirmishes were common and nearly 10,000 soldiers were killed even before the Korean War began.

The time seemed right for a Communist reunification of Korea - the Soviets had detonated their first nuclear bomb in September 1949; American soldiers had fully withdrawn from Korea; battle-hardened North Korean soldiers were returning from China’s successful communist revolution, in which the US had not intervened; and Stalin calculated that the US was focused on Europe. So, in April 1950, Stalin gave Kim Il-sung the go-ahead to invade the South if Mao would agree to send reinforcements, if needed. Stalin made it clear, however, that Soviet forces would not directly engage in combat with the US. 

Some 75,000 soldiers of the Korean People's Army (KPA) crossed the 38th parallel behind artillery fire at dawn on Sunday 25 June 1950. On 27 June 1950, two days after the KPA invaded and three months before the Chinese entered the war, President Truman had dispatched the United States Seventh Fleet to the Taiwan Strait, causing China to abort its planned invasion of Taiwan.  By 28 June, Seoul had fallen. Suffering severe casualties within the first two months, South Korean forces were pushed back to the Pusan perimeter.

The US then took advantage of the absence of the USSR from the Security Council to propose that the UN enter the war on the side of the South. The Soviet Union had walked out of the Security Council because the ROC and not the PRC was represented on the Council, and so was not present to veto the United States Security Council resolution authorizing military intervention on behalf of South Korea against the communist North.

In the US, the war was initially described by President Harry S. Truman as a "police action" as it was an undeclared military action, conducted under the auspices of the United Nations. The US would provide 88% of the 341,000 international soldiers who aided South Korean forces, with twenty other countries of the United Nations offering assistance. 

Command of UN forces was given to Douglas Macanthur, a Republican who publicly decalred his intention to to push the war into China so as to “liberate” that country from Communism, intending then return to a grateful country and run for President in 1952. 

On 15 September, 1950, a UN counter-offensive at Inchon (near Seoul) routed the North Koreans and drove them north of the 38th parallel and almost to the Yalu River, the border between North Korea and Communist China. Fearing Republican attacks that he was “soft on Communism (after all he had just “lost” China),  Truman gave MacArthur approval to proceed north of the 38th parallel. On 30 September, Zhou Enlai warned the U.S. that China was prepared to intervene in Korea if the U.S. crossed the 38th parallel. On 1 October 1950, the first anniversary of the founding of the PRC, the ROK Army crossed into North Korea; Kim Il-sung was sending frantic appeals to Mao for Chinese military intervention. From 2–5 October, Chinese leaders debated whether to send Chinese troops into Korea. Mao strongly supported intervention, and after Peng Dehuai citing MacArthur’s boast that he would “liberate China” made the case that US troops would likely reach the Yalu (the border between Korea and China) and invade China, the Politburo agreed to intervene in Korea. Meanwhile, on 7 October, with Truman’s approval, US forces crossed the 38th parallel. 

On 15 October 1950, President Truman and General MacArthur met at Wake Island in the mid-Pacific Ocean. This meeting was much publicized because of the General's discourteous refusal to meet the President on the continental US. MacArthur speculated there was little risk of Chinese intervention in Korea, and that the PRC's opportunity for aiding the north had lapsed. He believed the PRC had 100,000–125,000 soldiers at the Yalu River. He further concluded that, although half of those forces might cross south, "if the Chinese tried to get down to Pyongyang, there would be the greatest slaughter" without air force protection. 

On 25 October 200,000 Chinese troops entered North Korea. UN aerial recon had difficulty sighting PVA units in daytime, because their march and bivouac discipline minimized aerial detection. Marching only at night, the Chinese covered the 286 miles from An-tung, Manchuria, to the combat zone in 19 days.  

Chinese intervention forced the UN forces to retreat back to the 38th parallel. While not directly committing forces to the conflict, the Soviet Union provided material aid to both the North Korean and Chinese armies. The fighting ended on 27 July 1953, when the armistice agreement was signed. The agreement restored the border between the Koreas near the 38th Parallel and created the Korean Demilitarized Zone (DMZ), a 2.5-mile-wide fortified buffer zone between the two Korean nations.

Aftermath

Mao Zedong decided to confront the US first because he believed that such a military conflict was inevitable after the US entered the Korean War and second to improve his own prestige in the communist international community by demonstrating that his Marxist concerns were international. Within the PRC, the war improved the long-term prestige of Mao, Zhou, and Peng, and allowed the CCP to increase its legitimacy while weakening anti-Communist dissent. The Chinese war effort is considered an example of China's engaging the strongest power in the world with an under-equipped army, forcing it to retreat, and fighting it to a military stalemate. These successes were contrasted with China's historical humiliations by Japan and by Western powers over the previous hundred years. Mao had also discovered the usefulness of large-scale mass movements in the war while implementing them among most of his ruling measures over PRC. Finally, anti-American sentiments, which were already a significant factor during the Chinese Civil War, became ingrained into CCP culture during the Communist propaganda campaigns of the Korean War. The only significant negative consequence of the war for the PRC was that it led the US to guarantee the safety of Chiang Kai-shek's regime in Taiwan, effectively ensuring that Taiwan would remain outside of PRC control.