Battlefront Korea, Oct-Nov 1950 : Sensing complete victory, UN Security Council authorized US/UN forces to pursue Communist Korean remnants into North Korea. Intent was to basically complete what the Soviets and North Koreans started in Summer 1950, a forcible reunification of Korea, thereby ending the Communist Korean threat. By 26 Oct 1950, lead US elements pushed to Yalu River separating China and Korea.

Rugged mountain ranges separated eastern and western US/UN forces, hence a continuous line of contact could not be maintained during the advance. In this void, Chinese Communist volunteers began to filter into the region, hiding by day, traveling by night. Before long, about 250,000 Communist Chinese were in position for a counter-attack.

Although the recently formed US CIA reported alarming Chinese troop concentrations at the frontier, US General Douglas MacAuthur: supreme US/UN commander in Korea, dismissed this Chinese threat as mere extortion to prevent Korean unification. China was viewed as a paper tiger given her Peasant Army organization and logistically incapable of launching modern mechanized warfare. Douglas MacAuthur - en.wikipedia.org/wiki/Douglas_MacAuthur

Rollover : 25 Nov 1950, after a series of skirmishes between PLA and US/UN forces, Chinese People's Volunteer Army (CPVA) launched full counter-offensive along the entire Korean front with 270,000 men. Sheer enemy's volume overran, decimated, killed, and captured many Allied soldiers. US 1st Marine Division was quickly enveloped at Chosin Reservoir by 8 Chinese Divisions. CPVA did not possess heavy weapons, but the rugged Korean terrain forced the Marines to transverse the twisted mountain roads with their heavy equipment. This strategic withdrawal became the longest retreat in US military history, it was also the first retreat in US Marine history.

Marines withdrawal plan was to consolidate at HAGARU, fight their way southward to KOTORI, and linked up with US forces at HUNGNAM evacuation perimeter. Series of hilltop battles took place as CPVA setup artillery and ambush positions, thus forcing Marines to counter-attack to clear line of retreat. Marine Corsair units provided invaluable ground support in strafing, bombing, and napalming these hilltops. This was called Battle of the Chosin Reservoir - en.wikipedia.org/wiki/Battle_of_Chosin_Reservoir