Writing a comprehensive history of Polish citizens during the Holocaust is a hefty task. A Polish law that criminalizes any mention of Polish responsibility for or complicity in Nazi crimes makes it even harder. That makes the groundbreaking research conducted by acclaimed Holocaust historian Jan Grabowski for his new book, “On Duty: The Polish Blue & Criminal Police in the Holocaust” all the more remarkable. Relying on meticulous documentation, the book argues that Polish institutions were more than willing to assist the Nazis in their extermination campaign, and often led the way through their own initiatives.