He was never the “shadow president” that breathless liberals dubbed him, and perhaps not even the Great Manipulator of Time magazine’s dark and portentous cover. But if there’s a single person who has changed the American political conversation more than anyone else in the past year, it’s Steve Bannon.

Donald Trump blew into office with a basket of instinctive political beliefs—law-and-order, trade protection, anti-elitism—that felt incoherent in today’s American party landscape. They were immediately recognizable to Bannon, who had spent the past several years knitting those impulses into a new strain of right-populist politics at Breitbart News.

There’s no neat phrase to describe just what Bannon is—a Navy vet and former Goldman Sachs banker who transformed himself into a filmmaker and media entrepreneur. Most recently, he became a kind of intellectual Svengali to a diffuse group of people who might not have seen themselves as having anything in common until Bannon began articulating it, and gave it a tribune as chief executive of Trump’s campaign. Call it Trumpism, or Bannonism, but its anti-immigrant, anti-global, blue-collar nationalist worldview now contends with 1980s-style free-market constitutionalism for the soul of the Republican Party.

It’s hard to remember how different politics was just a year or two ago. “Nationalism” was a word nearly erased from the American mainstream, with dark echoes of the 1930s and the European right. Bannon revived it proudly, tweaking the phrase—“economic nationalism”—to give it less racist overtones. And he helped usher back a whole set of other ideas that haven’t been part of the American conversation for the past century.

More than any other figure in American politics, Bannon presents himself as a civilizational-level thinker, sometimes flamboyantly so. In his bleak worldview, America—and the West—are locked in a zero-sum struggle against sinister foes from Islamic jihadists to China to waves of immigrants threatening the values that built the nation. The collateral damage in that war often seems to include some of those values: pluralism, voting rights, an independent media, civil discourse.

Bannon’s legendary appetite for conflict hurt his ability to build influence in the West Wing and left him isolated before he was pushed out in August. And thanks to his boss’ lack of focus, much of his economic agenda—like a massive infrastructure program and populist tax reform—has never gotten off the ground. But Bannon’s fingerprints are all over some of Trump’s most controversial initiatives, like the travel ban. A question that has always swirled around Bannon is whether he’s just an opportunist fanning a dark but powerful American impulse that nobody else was willing to embrace openly, or whether a Harvard-educated former trader with a share of the “Seinfeld” royalties really does want to live in a more isolated, muscular country that has turned its back on the cosmopolitan culture that helped lift him to wealth and power. It may not matter: Donald Trump, of all people, is president of the United States; the country is locked in a struggle over what it really wants to be; and Steve Bannon is a big reason why. —Stephen Heuser

Q & A What’s the best book you read this year? Citizens: A Chronicle of the French Revolution, by Simon Schama What’s one out-of-the-box policy idea that we aren’t paying enough attention to? Dramatically lowering legal immigration. Twitter: Useful new tool or obnoxious distraction? Totally obnoxious—except when in the hands of the “Leader-of-Free-World.” Is Donald Trump draining the swamp? Yes. What historical moment does 2017 most resemble? 1854.