Final answer:
The Soviet Union was the first Allied nation to liberate a concentration camp, starting with camps in July 1944 and Auschwitz in January 1945, before the U.S. and British troops liberated other camps.
Step-by-step explanation:
The first Allied nation to liberate a concentration camp was the Soviet Union. The advancing Soviet armies began the process of liberating the death camps in July 1944, and they freed prisoners from Auschwitz, which was the largest of the Nazi death camps, in January 1945. These acts of liberation preceded the efforts by the U.S. and British forces that resulted in the freedom of prisoners held in Buchenwald, Bergen-Belsen, and Dachau in April 1945.
The liberation was a critical moment in revealing the horrors of the Holocaust, and Allied commanders such as Dwight Eisenhower ensured that the evidence of inhumanity was documented to counter any future claims of propaganda. The liberations were fundamental in bringing forward evidence for war crimes trials in the aftermath of World War II.