Fearing enemy infiltration, American troops stopped refugees streaming down the roads and told them to return home or stay in the hills, or risk getting shot by allied troops. On Jan. 14, 1951, the Army’s X Corps under Maj. Gen. Edward M. Almond ordered the “methodical destruction of dwellings and other buildings forward of front lines which are, or susceptible of being, utilized by the enemy for shelter.” It recommended airstrikes.

“Excellent results” was how American pilots summarized their strikes at Sansong on Jan. 19, 1951.

The same day, however, one of General Almond’s subordinates, Brig. Gen. David G. Barr of the Seventh Infantry Division, wrote to General Almond that “methodical burning out poor farmers when no enemy is present is against the grain of U.S. soldiers.” At least 51 villagers, including 16 children, were killed in Sansong, according to the Truth and Reconciliation Commission.

The attack on Tanyang followed the next day, when, survivors say, American planes dropped napalm near the entrance of the cave where refugees had sought shelter.

“When the napalm hit the entrance, the blast and smoke knocked out kerosene and castor-oil lamps we had in the cave,” Eom Han-won, then 15, said in an interview. “It was a pitch-black chaos  people shouting for each other, stampeding, choking. Some said we should crawl in deeper, covering our faces with wet cloth. Some said we should rush out through the blaze. Those who were not burned to death suffocated.”

Like Mr. Eom’s family, most of the people there were refugees who had been turned back at an American roadblock south of Tanyang, survivors said. In the days before the attack, the cave was packed with families. When the American warplanes flew in from the southwest, children were playing outside amid cattle and baggage.

That day, the Seventh Division’s operations logs noted that 13 planes attacked “enemy troops” and “pack animals and cave.” It reported “many casualties and got all animals.”

Mr. Eom, who rushed out of the cave into a hail of machine-gun fire from the planes but survived, said, “The Americans pushed us back toward the enemy area and then bombed us.” He said he lost 10 family members.