Republican Donald Trump skipped the debate where Fox News anchor Megyn Kelly co-moderated. Here's a look back at the clash that started with an earlier debate in August 2015. (Sarah Parnass/The Washington Post)

The Donald Trump-Megyn Kelly feud is now in its sixth month, and Trump just went nuclear.

The GOP front-runner's decision to boycott Thursday's final GOP debate before Monday's Iowa caucuses is a big one. Trump blames the decision to return Kelly to the moderator's table, though perhaps his motivation lies as much or more with the fact that debates haven't really helped him. Trump is good when the spotlight is on him -- not him and six or seven other Republicans.

But that aside, it's worth tracing this feud to its roots -- the questions Kelly asked Trump at the first Republican debate.

Kelly certainly came out with a doozy, asking Trump about his past derogatory comments about women. It was notable because that narrative hadn't really been a big part of the campaign to that point. And Trump clearly took exception to Kelly pointing it out.

Starting there, here are the questions Trump thought were so out-of-bounds:

1) "Mr. Trump, one of the things people love about you is you speak your mind and you don't use a politician's filter. However, that is not without its downsides, in particular, when it comes to women. You've called women you don't like 'fat pigs,' 'dogs,' 'slobs' and 'disgusting animals.' ...

Your Twitter account has several disparaging comments about women's looks. You once told a contestant on 'Celebrity Apprentice' it would be a pretty picture to see her on her knees.

Does that sound to you like the temperament of a man we should elect as president, and how will you answer the charge from Hillary Clinton, who was likely to be the Democratic nominee, that you are part of the war on women?"

First of all, Kelly has her facts correct here -- up to and including the "knees" part. Now, Trump could certainly make an argument that this comment wasn't intended to be as suggestive as some have said -- and as Kelly seemed to believe.

Judge for yourself here:

The rest of her question, while a little political process-y, is fair game. The GOP has been tagged as a party that struggles with female voters -- it regularly loses their votes and wins among men -- and it's logical to assume having a nominee who has said such things about women won't exactly help it in that regard.

2) "Mr. Trump, in 1999, you said you were, quote, 'very pro-choice.' Even supporting partial-birth abortion. You favored an assault weapons ban as well. In 2004, you said in most cases you identified as a Democrat. Even in this campaign, your critics say you often sound more like a Democrat than a Republican, calling several of your opponents on the stage things like 'clowns' and 'puppets.' When did you actually become a Republican?"

Again, Kelly's question here is blunt and meant to elicit an interesting response. But as we've noted before, the other candidates got the same kinds of questions:

Yes, the Fox moderators asked tough questions. Almost none of these questions, though, were surprising.

New Jersey has faced multiple credit downgrades under Chris Christie. It's kind of a big deal and would be if he were the GOP nominee. It would have been amazing if he hadn't been asked about it Thursday.

Similarly, Ohio Gov. John Kasich's decision to take the federal Medicaid expansion — something other GOP governors opted not to do — is also a no-brainer of a question. Ditto Jeb Bush's support for Common Core education standards and his support for a path to legal status for undocumented immigrants. Many of the "tough" questions were in this vein, giving the GOP contenders a chance to respond to their biggest hurdles to the GOP nomination. These were questions they were going to get eventually.

The total picture was one of tough questions framed in very pointed ways. That's what moderators are supposed to do. And lots of other candidates were on the receiving end.

For example, here's how Kelly asked Ben Carson about his foreign policy flubs: "Aren't these basic mistakes, and don't they raise legitimate questions about whether you are ready to be president?"

And here's her asking Scott Walker about his no-exceptions abortion stance: "Would you really let a mother die rather than have an abortion, and with 83 percent of the American public in favor of a life exception, are you too out of the mainstream on this issue to win the general election?"

And here's her asking John Kasich about his decision to expand Medicaid and his move to justify it by invoking Saint Peter's judgment at the Pearly Gates: "Why should Republican voters, who generally want to shrink government, believe that you won't use your Saint Peter rationale to expand every government program?"

These aren't the three most-pointed questions Kelly asked, by the way. These are the first three she asked after the first exchange with Trump.

And yet, Trump still feels like he was singled out. And here we are, five months later, with him refusing to even take the stage with her.

Businessman and Republican presidential candidate Donald Trump continues to defend himself after his controversial comments about Fox News anchor Megyn Kelly. (Alice Li/The Washington Post)