“He’s a fascist!” For decades, this has been a favorite smear of the left, aimed at those on the right. Every Republican president—for that matter, virtually every Republican—since the 1970’s has been called a fascist; now, more than ever. This label is based on the idea that fascism is a phenomenon of the political right. The left says it is, and some self-styled white supremacists and neo-Nazis embrace the label. But are they correct? To answer this question, we have to ask what fascism really means: What is its underlying ideology? Where does it even come from? These are not...