[16] On 19 October 1950, large formations of Chinese troops, dubbed the People's Volunteer Army (PVA), secretly crossed the border and into North Korea.

[37] The cold weather created considerable danger of frostbite casualties and was accompanied by frozen ground, icy roads, and weapon malfunctions.

[43] The mutual distrust between the commanders caused Smith to slow the 1st Marine Division's advance towards the Chosin Reservoir in violation of Almond's instructions.

[59] The food shortage forced the 9th Corps to initially station a third of its strength away from the Chosin Reservoir in reserve,[62] and starvation and exposure weakened the Chinese units, since foraging was not an option in the sparsely populated area.

[71] On the night of 27 November, the PVA 20th and 27th Armies of the 9th Corps launched multiple attacks and ambushes along the road between the Chosin Reservoir and Kot'o-ri.

[83] Aided by artillery from Hagaru-ri and Marine Corsair fighters, Fox Company managed to hold out for five days while enduring constant attacks by the PVA 59th Division.

[85] Believing that any further assaults would be futile, Song Shilun ordered the 9th Corps to switch their main attacks toward Sinhung-ni and Hagaru-ri,[85] leaving Yudam-ni alone from 28 to 30 November.

[86] At the same time, the US Eighth Army on the Korean western front was forced into full retreat at the Battle of the Ch'ongch'on River, and MacArthur ordered Almond to withdraw the US X Corps to the port of Hungnam.

[100] Although strong air cover suppressed most of the Chinese forces for the rest of the march, the cold weather, harassing fire, raiding parties, and roadblocks slowed the retreat to a crawl, while inflicting numerous casualties.

[102] RCT-31, later known as "Task Force Faith," was a hastily formed regimental combat team from the 7th Infantry Division that guarded the right flank of the Marine advance towards Mupyong-ni.

[103] Although the Chinese believed RCT-31 to be a reinforced regiment,[85] the task force was actually understrength, with one battalion missing, due to the bulk of the 7th Infantry Division being scattered over northeast Korea.

[110] Believing that the defenders had been completely destroyed at the inlet, the Chinese stopped their attacks and proceeded to loot the American positions for food and clothing.

[111][112] In the afternoon, Almond flew into the Sinhung-ni perimeter of RCT-31, convinced that RCT-31 was strong enough to begin its attack north and deal with whatever "remnants" of Chinese forces were in their way.

[135] As the front of RCT-31 made its way forward, heavy small arms fire caused many members of the rear guard to seek shelter below the road instead of protecting the trucks.

[145] Almost everyone, including rear support units with little combat training, was pressed into service on the front line due to the manpower shortage,[146] and the entire perimeter was on full alert by 21:30.

[175][176] In a last effort to stop the breakout,[175] the customary Chinese night attack was launched, with the 76th and 77th Division striking the Hagaru-ri perimeter from all directions.

[184] In desperation, Song Shilun ordered these troops to dig in at Funchilin Pass, while blowing up the vital bridge (40°13′55″N 127°17′46″E / 40.232°N 127.296°E / 40.232; 127.296), hoping the terrain and obstacles would allow the 26th and 27th Corps to catch up with the retreating UN forces.

[188] With the path to Hungnam blocked at Funchilin Pass, eight C-119 Flying Boxcars flown by the US 314th Troop Carrier Wing were used to drop portable bridge sections by parachute.

[209][212] According to official estimates by the People's Liberation Army General Logistics Department published in 1988, the PVA 9th Corps suffered 21,366 combat casualties, including 7,304 killed.

Outside of official channels, the estimation of Chinese casualties has been described as high as 60,000 by Patrick C. Roe, the chairman of Chosin Few Historical Committee, citing the number of replacements requested by 9th Corps in the aftermath of the battle.

[213] Regardless of the varying estimates, historian Yan Xue of PLA National Defence University noted that the 9th Corps was put out of action for three months.

According to a Defense Prisoner of War/Missing Personnel Office (DPMO) white paper, 1,394 names were also transmitted from the Chinese and North Koreans during the operation, of which 858 proved to be correct.

[224] Eliot A. Cohen writes that the retreat from Chosin was a UN victory which inflicted such heavy losses on the PVA 9th Corps that it was put out of action until March 1951.

[227] Appleman, on the other hand, questioned the necessity of a sea-borne evacuation to preserve the UN forces, asserting that X Corps had the strength to break out of the Chinese encirclement at Hungnam at the end of the battle.

[228] Chinese historian Li Xiaobing acknowledges X Corps' successful withdrawal from North Korea, and writes that the Battle of Chosin "has become a part of Marine lore, but it was still a retreat, not a victory.

[230] Patrick C. Roe, who served as an intelligence officer with the 7th Marine Regiment at Chosin,[231] asserts that X Corps directly allowed the Eighth Army to hold the south[m] and quoted MacArthur in corroborating his view.

[232] Zhang Renchu [zh], whose 26th Army was blamed for allowing the X Corps to escape,[6] had threatened suicide over the outcome, while Song Shilun offered to resign his post.

[240] The disregard by Far Eastern Command under MacArthur of the initial warnings and diplomatic hints by the PVA almost led the entire UN army to disaster at Ch'ongch'on River and Chosin Reservoir and only after the formation and stabilization of a coherent UN defensive line under Lieutenant General Matthew Ridgway did the "period of headlong retreats from an attacking, unsuspected foe" cease.

On 24 December 1950, MacArthur submitted a list of "retardation targets" in Korea, Manchuria and other parts of China, and requested 34 atomic bombs from Washington with the purpose of sowing a belt of radioactive cobalt to prevent further Chinese advances.

[245][246][247] The Battle of Chosin Reservoir is regarded by some historians as the most brutal in American history due to violence, casualty rate, weather conditions, and endurance.