Ceremonies in both Poland and Jerusalem marked the 75th anniversary of the liberation of Auschwitz, the death camp run by the Nazis that was synonymous with the Holocaust. Over a million people were murdered there between 1940 and 1945, the vast majority of them Jews.

Vice President Mike Pence and House Speaker Nancy Pelosi, Russian President Vladimir Putin, and French President Emmanuel Macron were among the leaders to attend the World Holocaust Forum in Jerusalem’s Yad Vashem Holocaust remembrance center.

The Nazis used slave labor to build a huge, sprawling prison complex and killing factory in occupied Poland outside Oświęcim. Entire families from across Europe were deported to Auschwitz-Birkenau in cramped cattle trains. Immediately upon arrival in Birkenau, the fittest were selected to work as slave labor, while others deemed too weak to work — older people, women, children, the sick — were immediately sent to die in the gas chambers.

The Allies first started receiving reports of what was happening in Poland and other death camps as early as 1942. But they disbelieved them as Polish propaganda. It wasn’t until the Soviet Red Army liberated Auschwitz on January 27, 1945, that the world first saw how systematic and horrible the Nazis truly were.