“Darkness is good,” was Bannon’s advice for dealing with criticism from groups such as the Anti-Defamation League. “Don’t let up.” At another moment, when the campaign feared House Speaker Paul Ryan would try to steal the G.O.P. nomination from Trump, Bannon threatened to rally Breitbart’s army. “Pepe’s gonna stomp their ass,” he said — “Pepe the Frog” being the alt-right’s white-supremacist cartoon mascot on Twitter.

Green is consistently interesting on the subject of Trump. But the real value of “Devil’s Bargain” is the story it tells about Bannon, some of which has been previously reported (not least by Green himself) but never so well synthesized or explained as it is here. The product of a working-class family and a Catholic military high school in Richmond, Va., he was taught from an early age that the defining moment in Western civilization occurred in 1492 — not with Columbus’s discovery of the New World, mind, but with Ferdinand and Isabella’s Reconquista from the Moors of the Iberian Peninsula.

“The lesson was, here’s where Muslims could have taken over the world,” recalled one of Bannon’s classmates. “And here was the great stand where they were stopped.”

If that was an early hint of Bannon’s political vision (and now a staple of Trump’s foreign policy speeches), other lessons suggested the means he would employ to achieve that vision. On Wall Street in the mid-1980s, he came to admire Michael Milken, the so-called junk bond king, who showed how a “band of outsiders” could set about “laying siege to a comfortable, fattened and vulnerable establishment.”

Later, while running an Internet business in Hong Kong, Bannon discovered the underworld of online gamers; “intense young men” who “disappeared for days or even weeks at a time in alternate realities.” One of those alternate realities was “World of Warcraft,” in which millions of people were digitally transformed into secret soldiers waging titanic battles in unseen worlds against mythical enemies.

Bannon seemed to intuit that this digital world could be recreated for his political purposes, by designing an apocalyptic narrative of righteous warriors waging an end-of-days battle by all necessary means against assorted enemies: jihadists, progressives, Acela-corridor Republicans, the Clintons. Republican political operatives had spent the Obama years wondering about the “missing” white voters who had failed to show up for John McCain and Mitt Romney. Turns out, they (or others like them) were online, and Bannon — whose own fantasies were suggested by a portrait he had of himself in his office, dressed as Napoleon — was proposing to supply this army with the necessary ammunition.

Much of it would come from the bile factory at Breitbart News. Another part would be supplied by the Government Accountability Institute, a Tallahassee, Fla.-based nonprofit that mined the “deep Web” and dug up the dirt on the Clinton Foundation for Peter Schweizer’s 2015 blockbuster “Clinton Cash.” There was also a data-analytics firm, Cambridge Analytica, an offshoot of a British company “that advised foreign governments and militaries on influencing elections and public opinion using the tools of psychological warfare.”