For most of the last 18 months, Donald Trump has been portrayed as a clown, a showman, an opportunist, a faux conservative, a political naïf, and an egomaniac bent on nothing but power and glory—but rarely as a man with an intelligible ideology. Yet if Trump’s ideas can’t quite be said to cohere into a unified worldview, and if his legion flip-flops deny him any claim to philosophical consistency, many of his signature promises and policies do add up to a set of ideas—populist, nationalist, authoritarian—with deep roots in American history.