NOEL KING, HOST:
Soon there will be another opening in President Trump's Cabinet. Secretary of Defense Jim Mattis sent the president a letter yesterday announcing his resignation. Just two years ago, Trump's enthusiasm for Mattis was pretty clear. When he picked him to lead the Pentagon, Trump called Mattis one of the most effective generals in U.S. history. So what happened?
NPR's White House correspondent Tamara Keith is with us now from the White House. Good morning, Tam.
TAMARA KEITH, BYLINE: Good morning.
KING: All right, so Trump and Mattis - these guys are two very different personality types. How did their relationship evolve over time?
KEITH: Well, President Trump was excited to have another general. He had a bunch of generals around him. He called Mattis Mad Dog, which is a nickname that Mattis doesn't really like, but he - but it is a nickname. It is his nickname. And, you know, President Trump didn't come from government, hadn't served in the military, was very deferential to the generals and including Mattis. And was - Mattis was able to persuade him on a number of things.
But over time, President Trump kind of didn't like it anymore. And in October, on "60 Minutes" - just a sign of how far their relationship came - President Trump was asked if Mattis might be leaving. And he said, oh, well, I have a very good relationship with him, but I think he's sort of a Democrat, which you could say is a kiss of death.
But in the end, it was Mattis who resigned in protest, telling the president that you have to have a secretary of defense whose views are better aligned with yours and arguing that the president's approach to the world order wasn't his approach.
KING: It was a really striking resignation letter. What does Mattis leaving mean for Trump's foreign policy team?
KEITH: You know, over time, President Trump could have been pushed away from some of his core beliefs. But eventually he came back to them, things like pulling out of the Iran nuclear deal, pulling out of the Paris climate accord, pulling troops out of Syria and reducing the number of troops in Afghanistan. So not having Mattis there - it's sort of a shortcut to the president's instincts.
KING: It also creates another Cabinet vacancy, which is not insignificant for this administration.
KEITH: That's right. So just going to throw some numbers at you - come the first of the year, after Mattis is gone in February, there are five Cabinet-level positions right now that require Senate confirmation to be filled again. And here are a couple of other numbers. President Trump is on his second secretary of state, his third national security adviser and will now be on his second defense secretary.
KING: In the few seconds we have left, the president's been tweeting this morning. What's he saying?
KEITH: He is saying that there's going to be a government shutdown if Democrats do not vote for border security, exclamation point. He is calling for the Senate to use the nuclear option to go around Democrats. And it's coming.
KING: Just a lot (laughter). NPR's Tamara Keith - thanks, Tam.
KEITH: You're welcome. Transcript provided by NPR, Copyright NPR.