I originally designed this study of fascism as a single chapter in a general book about the twentieth century, the third volume of my The Sources of Social Power. But my third volume still remains to be written, since fascism grew and grew to absorb my entire attention span over seven years. My “fascist chapter” was to be written first, since I was at that time spending a year in a Madrid institute with a fine library collection on the interwar struggle between democracy and authoritarianism. But then my research on fascism grew to the size of a whole book. I realized with a sinking heart (since this is not a pleasant subject on which to work for years) that it had to grow yet further. Since the deeds of fascists and their fellow-travelers culminated in mass murder, I had to engage with a second large body of literature, on the events centering on “The Final Solution” or “Holocaust.” I soon realized that these two bodies of literature – on fascists and their genocides – had little in common. Fascism and the mass murders committed during World War II have been mostly kept in separate scholarly and popular compartments inhabited by different theories, different data, different methods. These compartments have mostly kept them segregated from other rather similar phenomena of murderous cleansing that have been regularly recurring across the modern period – from seventeenth-century America to the mid-twentieth-century Soviet Union, to Rwanda-Burundi and Yugoslavia at the very end of the twentieth century.