This shows why the Marines were unable to rescue 7th Infantry Division's 31st RCT, which was destroyed only a few miles away, on the east coast of the Chosin Reservoir.
The Hagaru-ri valley was surrounded by commanding hills. Those vague shadows in the upper middle are still more and higher snow covered mountains. To put in a perimeter defense even on the near heights would mean manning about a four mile line, on the reverse slopes. This would take at least two infantry regiments. Lt. Col Tom Ridge, 3 Battn 1st Marines, had at his disposal for defense of Hagaru only two reinforced companies and a 6-gun 105mm battery.
With the few available tanks it was just possible to man a partial inner perimeter defense, on flat ground around Hagaru, and pray the enemy didn't have artillery to enfilade the camp. To weaken that defense force further by deploying units out of the perimeter in an attempt to relieve an Army force several times its size, while itself under attack by about the same size force attacking the Army RCT, would have been to risk the withdrawal route for the 5th and 7th Marines, and the whole of the First Marine Division.
In the event, there was much savage fighting just for East Hill, the immediate heights, before and after the 5th & 7th Marines reached Hagaru.
Reference: Chosin Reservoir
