In the east, on 27 November, the PVA 9th Army Group initiated the Battle of Chosin Reservoir. Here, the UN forces fared better: like the Eighth Army, the surprise attack forced X Corps to retreat from northeast Korea, but they were able to break out from the attempted encirclement by the PVA and execute a successful tactical withdrawal. X Corps established a defensive perimeter at the port city of Hungnam on 11 December and evacuated by 24 December, to reinforce the depleted Eighth Army to the south