On 25 November at the Korean western front, the PVA 13th Army Group attacked and overran the ROK II Corps at the Battle of the Ch'ongch'on River, and then decimated the US 2nd Infantry Division on the UN forces' right flank. The UN Command retreated; the U.S. Eighth Army's retreat (the longest in US Army history) was made possible because of the Turkish Brigade's successful, but very costly, rear-guard delaying action near Kunuri that slowed the PVA attack for two days (27–29 November). On 27 November at the Korean eastern front, a U.S. 7th Infantry Division Regimental Combat Team (3,000 soldiers) and the U.S. 1st Marine Division (12,000–15,000 marines) were unprepared for the PVA 9th Army Group's three-pronged encirclement tactics at the Battle of Chosin Reservoir, but they managed to escape under Air Force and X Corps support fire—albeit with some 15,000 collective casualties.
What organization was the US fighting with?
The UN