KELLY MCEVERS, HOST:

President Trump has ended a tumultuous week by saying goodbye to his chief strategist. Steve Bannon is out of the White House. Bannon has been credited with helping Trump win last year's presidential election. He's seen as a key architect of the president's anti-immigration and economic nationalism policies that appealed to white, working-class voters. It didn't take long for Bannon to return to running the right-wing website breitbart.com. He apparently chaired the evening editorial meeting there today.

Joshua Green wrote a book about President Trump's relationship with Steve Bannon, and I talked to him earlier today.

JOSHUA GREEN: I think the final straw was Bannon going out and publicly attacking not only rival members of the administration like Gary Cohn but publicly undercutting the president's foreign policy on North Korea. This came at a time when Trump's new chief of staff, General John Kelly, was trying to impose order and discipline on a chaotic White House.

But I think that the affront of having Steve Bannon, who'd been pushed out of the National Security Council, out there on the record, talking to the liberal policy journal The American Prospect and undercutting his own president was too much for even Donald Trump to put up with.

MCEVERS: Right. That was his conversation with The American Prospect. And he's saying basically that a military option with North Korea is not possible.

GREEN: Exactly. A lot of people privately would say what Steve Bannon said publicly, but you're not supposed to go out publicly and undermine your own president's negotiating position. I think it was that more than anything else that forced Trump's hand in firing Bannon.

MCEVERS: There was some speculation about that conversation he had with The American Prospect, though. Some people said he didn't know he was on the record. Knowing him the way you do, what do you think?

GREEN: You know, I know from my own reporting that he didn't think that he was on the record and thought that he was reaching out to someone who - Robert Kuttner, the author - who, while he may be liberal, had a hawkish stance toward China that reflected Bannon's own feelings.

And Bannon was really freelancing here on foreign policy and trying to put together some kind of an outside coalition that would pressure Trump's White House into adopting his preferred position on China. But this is precisely the sort of policy freelancing that Kelly was brought in to tamp down. Bannon had long been feuding with H.R. McMaster, the national security adviser, because they disagreed about foreign policy.

So Bannon had seemed like an ever-present fixture in Trump's political orbit. A lot of people, me among them (laughter), thought he could survive anything. But this turned out to be one step too far. And I think in the end, Trump really didn't have a choice.

MCEVERS: I think some people will see this, though, as also a reaction to the president's comments on Charlottesville when he, you know, said that both sides were to blame for the violence in Charlottesville. I think people, you know, credit Bannon with comments like that. And so letting him go might be a way to step away from those comments. What do you think about that?

GREEN: I don't think that analysis is correct. It's true that Bannon did not want Trump to apologize and was advising him to say exactly the sorts of things that Trump said, which seemed clear to me. But that is also viscerally and emotionally how Donald Trump feels himself.

MCEVERS: Right.

GREEN: And what was interesting to me is if you look at the history of the Trump-Bannon relationship, Bannon is often portrayed as a manipulator, but what he really is is an attack dog who will defend Trump no matter what. And sure enough, Bannon was really the only guy out there publicly defending Trump's response to Charlottesville. So it's surprising on one level to see Trump force out the one person in his own administration who was really standing up for him.

MCEVERS: How will the Trump White House change now that Bannon is gone?

GREEN: It's a great question, and I'm not sure we know. But Bannon was an important influence on Trump, who consistently was nudging him or pushing him towards the nationalist, populist policies that got him elected. I think with Bannon gone, there's going to be a power vacuum. There's some speculation that Bannon allies in the White House are also going to get pushed out.

What that leaves you with is really an inexperienced team of former Democrats, people like Jared Kushner, Gary Cohn, Steven Mnuchin, who don't necessarily have a set of policy ideas that is going to easily make its way through a Republican Congress. So I don't think we know. I mean I think of it in my own mind as being sort of a post-Iraq War situation where there is no central power, and the various fiefdoms will battle to take over whatever power and position Bannon is leaving open as he departs.

MCEVERS: Joshua Green is author of "The Devil's Bargain: Steve Bannon, Donald Trump, And The Storming Of The Presidency." And he is a correspondent for Bloomberg Businessweek. Thanks a lot.

GREEN: Great to be with you. Transcript provided by NPR, Copyright NPR.