Himmler’s orders served several purposes, according to research by the United States Holocaust Museum. First, he wanted to eliminate evidence of German crimes and witnesses who could testify to those crimes. He also hoped to use inmates as slave labor to keep the German war going. And rather irrationally, he believed that the prisoners could be used as bargaining chips in any peace negotiations.

While death might not have been the goal of the marches, that was indeed the fate of many, as the scattered gravestones that remain along these roads today attest.

Zofia Posmysz still remembers her inmate number: 7566. Sitting in her neatly kept apartment in Warsaw, the 96-year-old survivor remembered the biting cold on the night the guards gathered thousands of women outside the gates of Birkenau, a death camp that was part of the Auschwitz complex.

“We didn’t know what it meant that we would leave the camp,” she said. “We didn’t know if we would have to undergo some sort of selection.

“We heard that those who could not walk would get to stay in the hospital, but we weren’t sure if they would be kept alive. We knew nothing and worried.”

But how could it be worse than the hell she had endured for three years? One memory came rushing back to her.