The Hungnam evacuation was a notable event during the early years of the Korean War. Retreating to the port of Hungnam from the recent defeat at Chosin Reservoir, UN forces managed to evacuate about 100,000 troops and about 86,000 refugees to South Korea, in what's also known as the Miracle of Christmas. This was partly due to the lack of any large scale enemy attacks.
Why didn't the PVA or KPA attack during the evacuation? The source that Wikipedia uses, Mossman, Billy (1988). United States Army in the Korean War: Ebb and Flow November 1950-July 1951, offers these explanations:
None of the enemy strikes on the perimeter did more than penetrate some outposts, and counterattacks rapidly eliminated these gains. So far, all action appeared to be only an attempt to reconnoiter the perimeter. Several explanations for the enemy's failure to make a larger effort were plausible. The bulk of the Chinese in the Changjin Reservoir area apparently were taking time-probably forced to take time-to recuperate from losses suffered in the cold weather and recent battles. All enemy forces undoubtedly were aware that the X Corps was evacuating Hungnam and that they would be able to enter the city soon without having to fight their way in. The contraction of the corps perimeter probably forced the enemy to repeat his reconnaissance. Artillery fire, naval gunfire, and ample close air support may well have prevented the enemy from concentrating sufficient strength for strong attacks. Whatever the reasons, enemy forces had not yet launched a large-scale assault.
- The Chinese were recuperating from losses
- The rapid contraction of the defensive perimeter forced repeated reconnaisance
- Fire support prevented the enemy from gathering forces
These are plausible - particularly since the 9th Army Group was badly mauled during the previous battle - being "put out of action for three months". Nevertheless, these are conjectures, and I was curious if there's more solid evidence for the lack of large scale attack.