Over two days in September, 1941, German soldiers, assisted by Ukrainian collaborators, murdered 33,771 Jews at the Babi Yar ravine outside Kyiv. The massacre was one of the earliest and deadliest episodes in what is sometimes called the “holocaust by bullets,” a phase of the Nazi genocide that took place outside the mechanized slaughter of the death camps. These mobile killing squads, known as Einsatzgruppen, are estimated to have taken at least 1.5 million lives.