I think the destruction of the Army 31st RCT on the east side of the Chosin Reservoir was the most devastating loss due largely to high command incompetence and gross negligence.
Only 385 doggies out of 3,000 escaped to the 1st Marine perimeter where Chesty Puller threatened to have his tanks fire on them if they withdrew as much as a foot from their assigned positions.
He joined the Army when he was seventeen and was sent to the Philippines in July, 1941. Eight months later, he was captured by the Japanese and sent to a copper mine in Korea, where he was assigned to the blasting team. He quickly found a way to keep the Japanese from being too rough on him; he acted like he was crazy. The guards were convinced that his "insanity" was a punishment from the gods and they did not want to bring the wrath of those gods down upon themselves, so they generally left him alone.