At the end of the war when it became obvious that the German army was trapped between the Soviets to the east and the advancing Allied troops from the west, the Nazis, in an attempt to prevent the liberation of camp inmates, forced them to march westward. Thousands died in these marches.
When Russian armies moved in from the East, the concentration camps were taken apart and the prisoners marched to Germany, in camps such as Bergen-Belsen. A third of the prisoners died in January 1945.
When the Allied Forces began to enter Germany and other Nazi occupied land, the Nazis emptied the camps to wipe out any evidence. A few surviving Jews were forced to walk on the death marches out of the camps.