(CNN) President Donald Trump has revived one of his most significant false claims from the 2016 presidential election: his inaccurate insistence that he opposed the 2003 invasion of Iraq.

The claim was definitively debunked in 2016 by Andrew Kaczynski and Nathan McDermott of CNN's KFILE team, who were then reporting for BuzzFeed. But Trump has started repeating his fiction again -- at least four times since August, including in his remarks on Sunday about the death of ISIS leader Abu Bakr al-Baghdadi

So it's worth laying out what actually happened.

Contrary to his repeated claims, Trump did not publicly express opposition to the invasion of Iraq before it occurred . He began criticizing the war in 2003, after the invasion, but he also said that year that American troops should not be withdrawn from Iraq. He emerged as an explicit opponent of the war in 2004.

What Trump has said in the last two months

Trump has been claiming not only that he opposed the invasion, but that he was vocal about this supposed opposition.

Trump told Fox News Radio on August 29 : "Look, we should have never been in Iraq. I've said that from day one and I was a civilian and it was covered but it was -- you know, I was a civilian so who cares, right? But I said from day one, we should not go to Iraq."

Speaking to reporters on September 11 , Trump said, "I disagreed with that decision from the beginning, even though I was a civilian, so nobody cared. But I was out there. I was outspoken about it. I thought it was a terrible mistake."

Trump repeated a similar comment at the Values Voter Summit on October 12 . And on Sunday, Trump said , "If you read about the history of Donald Trump -- I was a civilian. I had absolutely nothing to do with going into Iraq, and I was totally against it." He added: "President Bush went in. I strongly disagreed with it, even though it wasn't my expertise at the time, but I had a -- I have a very good instinct about things."

What Trump actually said before the war

In his 2000 book "The America We Deserve," Trump argued that a military strike on Iraq might be necessary.

Trump wrote that the US still did not know the true status of Iraq's nuclear program. He then wrote, "I'm no warmonger. But the fact is, if we decide a strike against Iraq is necessary, it is madness not to carry the mission to its conclusion. When we don't, we have the worst of all worlds: Iraq remains a threat, and now has more incentive than ever to attack us."

He continued: "Am I being contradictory here, by presenting myself as a deal-maker and then recommending preemptive strikes? I don't think so. There's nothing really comparable to unleashing a squadron of bombers, but in the world of business sometimes you have to make quick, secret, decisive moves in order to gain a negotiating advantage."

When radio host Howard Stern asked Trump in September 2002 if he is "for invading Iraq," Trump responded, "Yeah, I guess so. I wish the first time it was done correctly."

Trump did not express a firm opinion about the looming war in a Fox interview in January 2003 , saying that "either you attack or don't attack" and that President George W. Bush "has either got to do something or not do something, perhaps."

What Trump said during the war

The day after the invasion in March 2003, Trump told Fox : "It looks like a tremendous success from a military standpoint."

Trump started publicly expressing negative or skeptical thoughts about the war shortly after it began. He called the war a "mess" in a brief comment at an Academy Awards after-party later in the week of the invasion. Six months into the war, Trump said, "It wasn't a mistake to fight terrorism and fight it hard, and I guess maybe if I had to do it, I would have fought terrorism but not necessarily Iraq."

Still, Trump did not express definitive opposition in 2003. In a MSNBC appearance in November 2003, he lamented US spending on the war and said that "the question is whether or not we should have been in Iraq in the first place" -- but he continued: "I don't think that this president can do anything about that. He is really -- he is on a course that has to stay."

In December 2003, Trump told Fox that the war had been "tougher than people thought," but he added, "It just seems to be something that, we are there now, we have to stay, we have to win, otherwise we just won't have the same respect."

It wasn't until more than a year after the invasion that Trump conveyed explicit opposition to the war.

He told Stern in April 2004 that "Iraq is a terrible mistake." Trump was quoted in Esquire magazine in July 2004 lamenting deaths and injuries among Americans and Iraqis, saying, "And it turns out that all of the reasons for the war were blatantly wrong. All this for nothing!"

What Trump said about 'the oil'

In his remarks on Sunday, Trump added a new detail to his story.

Defending his decision to leave some American troops in Syria to "secure the oil," Trump said that he had argued in favor of seizing oil from Iraq at the same time as he was opposing the 2003 invasion.

"I always used to say, 'If they're going to go in...' Nobody cared that much, but it got written about. 'If they're going to go in...' I'm sure you've heard the statement, because I made it more than any human being alive. 'If they're going into Iraq, keep the oil.' They never did. They never did," he said.

We could not find any examples of Trump saying anything before the war about keeping Iraq's oil. (We've asked the White House if they can provide any evidence; we'll update this article if we receive more information.) As with his stance on the invasion itself, Trump appeared to be describing comments he made during the war as if he made them during the run-up to the war.

There is at least slightly more factual basis for this claim than his claim to have opposed the invasion: Trump did have a history of suggesting that the US get or take Middle Eastern countries' oil.

In a CNN appearance in 1987 , for example, he called for the US to get a percentage of Kuwait's oil in exchange for military protection. The same year, Florida's St. Petersburg Times reported that Trump had suggested in a New Hampshire speech that "the United States should attack Iran 'and take over some of their oil.' "

But these comments were more than 15 years before the invasion of Iraq, and they were not about Iraq.

Trump was more explicit by 2010. Criticizing the Obama administration in another interview with Blitzer, Trump said, "We weren't smart enough to take the oil."