Final answer:
The 'dead and walking dead' refers to the emaciated survivors and corpses that Allied troops found upon liberating the concentration camps, illustrating the extent of Nazi atrocities during the Holocaust.
Step-by-step explanation:
When Oporah mentions that when American and Russian troops liberated the camps "all they found left behind were the dead and the walking dead," it refers to the horrific scene soldiers encountered upon entering concentration and extermination camps at the end of World War II. The term "walking dead" describes the survivors who were so emaciated and weak from starvation and mistreatment that they appeared lifeless - mere shadows of living human beings. The liberation of these camps revealed the extent of the Nazi atrocities and the inhuman conditions that the Jewish prisoners and other victims endured, which included being forced to march for miles in brutal winter conditions and being systematically exterminated.