President Trump devoted much of his “Fox and Friends” interview Tuesday morning to deriding his perceived foes and, in particular, to combating Bob Woodward’s new book.

But even as he called into question the book’s credibility, he reinforced his own lack thereof — and all you have to do is compare what he said Tuesday to the last time Woodward published a book.

In the Fox interview, Trump criticized former defense secretary Jim Mattis, who has in recent months warned the country strongly against reelecting Trump. But in the course of making that case, Trump offered an odd claim: He said Mattis had effectively stood in the way of his efforts to assassinate Syrian President Bashar al-Assad.

“I would’ve rather taken him out,” Trump said. “I had him all set. Mattis didn’t want to do it. Mattis was a highly overrated general.”

When asked whether he regretted not taking Assad out, Trump added: “No, I don’t regret that. … I had a shot to take him out if I wanted. Mattis was against it.”

Then-defense secretary Mattis said he did not rule “anything out” when asked whether he'd take action over a suspected chemical attack in Syria in 2018. (The Washington Post)

The first problem with this argument is that Trump is disparaging Mattis for opposing something that Trump doesn’t even say he regrets. The second is that the commander in chief makes these decisions, full stop. If Trump wanted to do it, Mattis couldn’t block him. The fact that Mattis expressed opposition to a plan that Trump even today doesn’t necessarily advocate may not be the best example of him being a bad public servant.

But perhaps the biggest problem is that, in the course of making this strange argument, Trump directly contradicted himself. The contradiction dates back, as it happens, to the last time Woodward published a book about him.

In 2018, Woodward published “Fear.” In the book, he reported Trump had considered assassinating Assad.

After Assad launched a chemical attack on civilians in April 2017, Trump called Mattis and said he wanted to assassinate the dictator. “Let’s fucking kill him! Let’s go in. Let’s kill the fucking lot of them,” Trump said, according to Woodward.

Trump, on Sept. 5, 2018, flatly denied that.

“I heard somewhere where they said the assassination of President Assad by the United States. Never even discussed,” Trump said, adding: “No, that was never even contemplated, nor would it be contemplated.”

He even held it up as evidence that the book shouldn’t have been published.

“It should not have been written about in the book,” Trump said. “It’s just more fiction. The book is total fiction. Okay?”

Trump is now confirming what he claimed was “fiction” was actually very accurate. But as is often the case with Trump, it’s still not clear what is the truth.

Trump in 2018, after all, wasn’t the only one offering something amounting to a denial. So, too, did then-United Nations Ambassador Nikki Haley.

“I have the pleasure of being privy to those conversations … and I have not once heard the president talk about assassinating Assad,” Haley said at the time.

It’s theoretically possible such discussions occurred after the September 2018 denials, but Mattis resigned in December of that year. In the intervening three months, there were no new chemical weapons attacks by the Assad government. Trump was also moving to withdraw from Syria at this time — a decision that led to Mattis’s resignation.

The White House had good reason to deny such plans in 2018. Even planning such an operation as a contingency would be highly questionable, given its impact in a volatile region — a fact that reinforces the dicey nature of what Trump decided to confirm Tuesday morning. But after all that went into Trump’s denial, we just needed Woodward to publish another book for Trump to give an entirely different version of events.

It sure undermines Trump’s efforts to cast Woodward as the one who can’t be trusted.