The conflict has been there for years, starting back on June 25, 1950. Strictly speaking, it was not a “new” war, but simply an extension of the Second World War, in which Stalin was determined to close the circle with Japan, with which the Soviet Union had never concluded a peace treaty. In fact, the Soviet Union only joined in the war against Japan in the closing days, and was not included in the signing of the surrender of the Japanese Emperor to General MacArthur. In the fall of the dominoes with China and much of southeast Asia, and a good possibility that Japan would be next, the North Koreans told Stalin they could take all of the South Korean peninsula. And they almost succeeded. Only a small beachhead around Pusan was still in the hands of the US and its allies, when the landing at Inchon virtually cut the North Korean army in two, and the push came on to drive the North Koreans back, with US Marines arriving at the Chosen Reservoir by November 1950.
But they had outrun their supply lines, and one of the bitterest winters in years hit, leaving the Marines with inadequate clothing and diminishing ammo. No evacuation route was available, and many simply froze to death. Not only that, but the swarm of Chicom soldiers over the Yalu River pretty much rescued the remnant of the North Korean army and government, with the resulting tug-of-war at what became the Line of Demarcation and agreed-upon DMZ.
Also recall, that Truman fired MacArthur, and that alone probably made any US and allied forces victory impossible.
Read “The Marines of Autumn.” Fantastic. Americans should have a daily moment of silence for those men.
MacArthur assumed the NK’s were beaten and that rumors of a Chinese Invation were overblown and therefore incorrect. He had his units spread out an moving fast toward the Yalu in an attempt to secure the territory and accept surrender of any NK units encountered. The Army wasn’t expecting major combat, but the Marines were. General Smith kept the regiments of the 1st MarDiv as closed-up as possible under the circumstances (a road march on a single axis of advance). MacArthur’s staff was hounding him to speed up the pursuit. Smith didn’t see it that way and that more than anything probably allowed the 1st MarDiv to survive.