Republican presidential candidates have been lining up to call it an error.
Ted Cruz, Scott Walker, Marco Rubio, George Pataki, Rick Santorum, Rand Paul, radio host Laura Ingraham — every one took a position that would have been anathema within the party a few years ago.
It was all triggered by an apparent flub by Jeb Bush.
The presidential hopeful, ex-Florida governor and brother of the president who attacked Iraq was asked on Fox News last week whether he would have invaded, knowing what he knows today.
Yes, he replied.
He tried to correct the record a day later. He said he'd misinterpreted the question and had based his answer on how he felt in 2003 — when the U.S. government said Saddam Hussein had weapons of mass destruction and an invasion would be quick and easy. By week's end, he settled on a final answer: no.
He was pounded with criticism in the meantime — largely from his own side.
Ingraham, a popular conservative radio host, said: "You can't still think that going into Iraq, now, as a sane human being, was the right thing to do. If you do, there has to be something wrong with you."
Other presidential aspirants had no problem answering the Iraq question.
"Of course not," Cruz, the conservative favourite from Texas, told CNN.
"I mean, the entire predicate for the war against Iraq was the intelligence that showed they had weapons of mass destruction and that there was a real risk they might use them....
"We now know that intelligence was false."
Like most Republicans, Sen. Marco Rubio tried to avoid blaming George W. Bush. He said in an interview that Bush made the right decision — based on intelligence he had at the time. But Rubio added: had everyone known the intelligence was wrong, neither Bush nor Congress would have authorized the war.
Bush admitted in his post-presidential memoir in 2010, "Decision Points," that he'd sent American troops into combat based in large part on bad intelligence.
"That was a massive blow to our credibility — my credibility — that would shake the confidence of the American people," said the ex-president. "I had a sickening feeling every time I thought about it. I still do."
Sen. Rand Paul stood out from the pack.
An intervention-wary libertarian, Paul said he not only regrets the war today, but did so even back then. That puts him to the left of Democratic rival Hillary Rodham Clinton on the issue and in tune with another Democrat whose view was once considered outside the mainstream — Barack Obama.
Republicans now argue the Iraq war on new grounds.
They blame Obama and Clinton for retreating in 2011, leaving an unstable country riven by cultural hatreds and ripe for a takeover by Islamist rebels.
Polling shows a drastic evolution in American public opinion. In 2003, Gallup found 91 per cent of Republicans supported the invasion. Asked whether the war was a mistake, 72 per cent of overall respondents disagreed. By June 2014, only 39 per cent disagreed it was a mistake.
Matthew Dowd, George W. Bush's chief strategist in 2004, described his own shifting views. He wrote about driving to the airport at the time with his son, a soldier, and then to the Bush campaign headquarters.
"I had just put my son in the hands of the army and was about to help re-elect the person who would ultimately send him into harm's way," said Dowd, now a political commentator at ABC.
"I have come to the place where I believe the decision to go into Iraq was a huge mistake. Should have I asked more questions in the midst of all that? Yes."