On this Holocaust Remembrance Day, historian Dr. Dan McMillan, author of "How Could This Happen: Explaining the Holocaust," spoke with WCBS 880 Anchor Steve Scott.

PHOTO: A view of barbed wire fence and surveillance towers at the former Auschwitz Birkenau site on January 27, 2022 in Oswiecim, Poland. The main theme of the 77th anniversary will be the beginning of the extermination in the German Nazi camp Auschwitz, which took place in the spring of 1942. The Nazis killed an estimated one million people at the camp during the World War II occupation of Poland by Nazi Germany. The Soviet Army liberated the camp on January 27, 1945. (Photo by Omar Marques/Getty Images)