Former GOP presidential candidate Ben Carson is now President Trump's housing and urban development secretary. Here's what you need to know about the neurosurgeon. (Sarah Parnass,Osman Malik/The Washington Post)

The decision by Donald Trump to appoint Ben Carson to his Cabinet closes the circle on a very, very weird relationship — and an even weirder flirtation between the two men over the past several weeks.

Carson rose to national fame in 2013 after criticizing President Obama during a National Prayer Breakfast. Although that mirrors the way Trump made his political bones (attacking Obama's eligibility), it seems a bit out-of-character for the former neurosurgeon. Trump is known as a rabble-rouser; Carson is described as “soft-spoken” so regularly that he might as well have written it on a baseball cap and worn it across the country. Carson and Trump were personally different in every possible way, save one: Neither had any experience in government service. That was enough to propel Trump into the lead by mid-July 2015 and for Carson to separate himself from the pack by early September that year.

The two got along at first. In August that year, Carson said that, sure, he might be willing to consider being Trump's vice president. The two signed a joint letter in October, asking CNBC to put a hard time limit on a debate the network was hosting. Two outsiders, allied for change.

Until Carson tied Trump nationally and then passed him in Iowa. “Carson and Trump are dominating,” our headline at the time read, “but their chummy rapport turns cool.” This was the point at which Trump started referring to Carson as “sleepy,” at one point suggesting that Carson might actually be lower-energy than — gasp! — Jeb Bush. ("I have plenty of energy,” Carson replied, though he admitted that “I do have a tendency to be relaxed.")

This was the pattern with Trump. Anyone who he felt threatened his chances for success became Public Enemy No. 1. First it was Bush. Then Carson. By January, Ted Cruz. And in the general-election cycle, Hillary Clinton.

Trump's sharpest attack on Carson came in early November, a year before the general election and right as Carson was tying him in national polls. Carson's best-selling biography had come under fire, especially a story in which Carson claimed to have tried to stab a friend with a knife only to see the blade glance off his friend's belt buckle.

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At first, Trump's attacks were mostly on Twitter, as is his way. He called Carson a liar after reports emerged that Carson had claimed to have been offered a scholarship that was never on the table.

At a rally in Iowa, Trump mocked Carson's biography, flipping his belt back and forth to demonstrate how difficult it might be for a knife to have been deflected. He suggested that the story indicated Carson might suffer from pathological issues, which, like the psychology of a child molester, he said, could not be cured. He asked the audience to not “be fools” by supporting Carson over him.

“How stupid are the people of Iowa? How stupid are the people of the country to believe this crap?” Trump said of Carson's claims.

Carson responded by saying Americans were “sick and tired of” attacks like Trump's.

“Now that he's completed his gratuitous attack, why don't we press on and deal with the real issues,” Carson told reporters. “That's what the people of America are concerned about, not so much politics as usual, politics of personal destruction. That's what the American people are sick and tired of.”

Trump later claimed that it was this speech that demolished Carson's position in the polls. The doctor sank from first to fourth nationally over a month-and-a-half leading into December, according to the RealClearPolitics average of polls. It's more likely though that the blame lay with the terrorist attacks in Paris. Foreign policy was a weakness for Carson, and in the wake of the attacks, he evaporated.

He was no longer a factor in the Republican contest from that point on, though he stuck around after coming in fourth in Iowa. Trump turned his fire elsewhere. Carson dropped out after Super Tuesday.

Less than a week later, Carson was on Team Trump, offering the businessman a rather odd endorsement.

“There's two Donald Trumps,” he said. “There’s the Donald Trump that you see on television and who gets out in front of big audiences, and there’s the Donald Trump behind the scenes. They’re not the same person. One’s very much an entertainer, and one is actually a thinking individual.”

Before another week passed, Carson doubled down on his faint praise. If Trump “turns out not to be such a great president,” he told NewsMax TV, “we're only looking at four years.” This is one reason that our Ben Terris declared Carson to be “the best or worst surrogate of all time.”

There were some brief rumblings that Trump might pick Carson as his vice president after Trump won the nomination. This appears to have never been terribly serious, along the lines of many of the other numerous people whose names were at some point floated. But even before Trump's unlikely victory earlier this month, Carson was being discussed as a possible member of a Trump Cabinet.

A few options were mentioned. When Carson endorsed, Trump hinted at giving him a role with the Departments of Education or of Health and Human Services. Last month, Circa reported that Carson had been offered the Health and Human Services post but that he had declined. That information came from Armstrong Williams, Carson's longtime business manager.

“Dr. Carson feels he has no government experience, he's never run a federal agency,” Williams said to the Hill when explaining why Carson passed. “The last thing he would want to do was take a position that could cripple the presidency." (It was quickly pointed out on social media that Carson had, however, considered himself eligible to be president.)

Carson told The Washington Post's Robert Costa then that he didn't plan to work within the administration.

“The way I'm leaning is to work from the outside and not from the inside,” he said. “I want to have the freedom to work on many issues and not be pigeonholed into one particular area.”

He added, “My view is that if some people and the media are going to hate him, then he's going to need allies on the outside to be there, to be there to move the country forward. I don't care about a position.”

Around Thanksgiving, Carson hinted that this position had changed, in a post on Facebook. And then, on Monday, the announcement: Ben Carson would be nominated to serve as the secretary of Housing and Urban Development. His qualifications for the gig? Well, that's not entirely clear, beyond the fact that Carson grew up in inner-city Detroit. (During Trump's halfhearted effort to reach out to the black community during the general-election campaign, Carson showed him around his childhood neighborhood.)

Carson, Trump said in the Monday statement announcing the pick, was a “tough competitor” who “never gives up.” For the most part.