"Bolton is extremely famous for his fervent hawkery, including on the Iraq war. If Trump bothered to do a cursory Google search on Bolton before appointing him..."

"... to the most powerful national security position in his administration, he’d have turned up headlines like 'John Bolton: No regrets about toppling Saddam.' Sadly, there was too much good stuff [on] television in the days leading up to Bolton’s nomination to do that search. Trump does not seem to realize how bad it makes him sound that he never bothered to ask what he later identified as the key question about the worldview of his own national security adviser."

From "Trump: I Didn’t Realize Bolton Supported Iraq War Until After I Hired Him" by Jonathan Chait (New York Magazine). Chait is reading the WSJ interview in which Trump says:

He had a lot of policy disputes, he and I. And after the first month or so, you know, I asked him one question. I said, “So, do you think you did the right thing by going into Iraq?” He said, “Yes.” And that’s when I lost him. And that was early on. That’s when I lost him. But no, I disagreed with much of the stuff he said. He was one of many people. I liked listening to many people, and then doing whatever is the right thing to do.

You didn’t ask him about Iraq before you brought him into the White House? If he regretted that?

No, but it didn’t … I knew all about his policy on Iraq. But that didn’t matter, frankly. Because he made a terrible mistake. And so did everybody else involved in Iraq and the Middle East, frankly. I never thought it was the right thing to do. And I’ve been proven right. But when he told me he still thinks it was the right thing to do, and was unable to explain it to me, I said, “Explain that to me, because I don’t think you can.’ And he could not explain it to me. So I said, “Do you say that just to make yourself feel good? Or do you say that because you really believe it?” He said, “I really believe it.” I said, “Well, then you’ve lost me because it’s just wrong.”...

[W]hen I asked him the question, so John, you were one of the people that were really pushing hard to go into the Middle East, to go into Iraq. Would you do it again? He said, Yes. And that’s where I said this guy is crazy.... I was talking to him. I said, So was that a mistake? I said, and it’s okay to admit you made a mistake, although that’s a big one. That’s a beauty. And I said, Do you think it was a mistake? And he said, No, I think it was the right thing to do. And I said, You know, you can’t explain that. You just can’t explain it.

Why didn't Trump ask Bolton before he was hired whether in retrospect he still thinks it was the right decision to go into Iraq?  Chait's answer is that that Trump is impulsive and reckless: He just didn't "bother." A more charitable reading of Trump — and I'm not saying the President deserves charity, just trying to balance things a little — is that he'd formed the opinion that everyone knows now that the Iraq War was a mistake. Trump was and is very proud of his opposition to the Iraq War, his astute perception from the beginning that it was a mistake. But he lacked the astute perception to see that there were still some people who believed the war was a good idea and to notice he was hiring one of those people.

You know this morning when I saw this tweet of Trump's...

Bolton’s book, which is getting terrible reviews, is a compilation of lies and made up stories, all intended to make me look bad. Many of the ridiculous statements he attributes to me were never made, pure fiction. Just trying to get even for firing him like the sick puppy he is!

— Donald J. Trump (@realDonaldTrump) June 18, 2020

... I was going to snark You knew he was a snake.... you know Trump and that song lyric he's recited many times about the woman who takes in and nurtures a snake that ultimately bites and kills her?

“Oh shut up, silly woman,” said the reptile with a grin

“You knew damn well I was a snake before you took me in

But I guess Trump didn't know Bolton was a snake.