Most people expected it at some point. They thought it would come after he attacked Senator John McCain.
He dismissed his five years in a North Vietnam prison, saying the one-time Republican candidate wasn't a hero, and he preferred people who weren't captured.
And then they thought it might be during the first candidates' debate. Or the second. Or the third, when he was no longer leading in all the polls. Or the fourth.
But it didn't.
Yet, in a 95-minute tirade on a stage in Iowa, Donald Trump's meltdown moment may finally have come.
They'd lined up in the gusty winds and the dropping temperatures in Fort Dodge. They'd bought the "Make America Great Again" hats; they'd brought their Trump books to be signed. And they'd waited. And they'd waited. Local endorsers spoke, but the crowd really wanted to see the man himself.
He turned up late. They know he rarely talks from a script. But what followed was described by one journalist as "an unfocused rant which felt like it might never end".
His main attack was on Ben Carson. The neurosurgeon is leading in many polls, and, crucially, he leads in Iowa.
What Trump fails to appreciate is that voters in the state are attracted to Carson spiritually and politically.
And with many evangelical Republicans driving the caucus in February - the first electoral test of the Republican field - he commands more loyalty than Trump.
Talking about Carson's biography, where he wrote that he tried to stab a friend only for his belt buckle stopping the blade, Trump blasted: "How stupid are the people of Iowa? How stupid are the people of this country to believe this crap?"
It didn't end there.
Saying Carson's behaviour was "pathological", Trump ramped up the toxicity of his attack: "If you're pathological, there's no cure for that, folks.
"There's no cure for that … If you're a child molester, a sick puppy, you're a child molester, there's no cure for that. There's only one cure; we don't want to talk about that cure. That's the ultimate cure.
"Well, there's death, and there's the other thing. But if you're a child molester, there's no cure. They can't stop you. Pathological, there's no cure."
Having compared his main rival to a child molester, and the people who might vote for him as stupid, Trump wasn't done.
Talking about Marco Rubio's position on immigration, Trump described the Florida senator as "weak, like a baby, like a baby" and "not a good poker player because every time he's under pressure he starts to profusely sweat".
Trump said Jeb Bush, the former Florida governor, didn't deserve his attention because his campaign was doing so poorly. He also slated two other candidates: Ohio Governor John Kasich and businesswoman, Carly Fiorina.
He attacked Democratic front-runner Hillary Clinton too, insisting she was playing "the woman's card, big time".
And suggesting he would bomb "the s**t" out of areas controlled by the Islamic State of Iraq and the Levant (ISIL) - or ISIS as he called it - he insisted: "I know more about ISIS than the [US military] generals do. Believe me."
'Dangerous and divisive'
People like Trump's anger, his disenchantment. It chimes with the political mood in the country.
One Washington Post reporter who was there wrote that the crowd initially laughed and cheered and applauded, thinking this was simply vintage Trump, the show they'd come to see.
But as the speech dragged on, and the attacks became more personal, she wrote that "the audience grew quiet, a few shaking their heads. A man sitting in the back of the auditorium loudly gasped."
Senior figures in the Republican Party have been worried Trump might actually win the nomination.
They see him as dangerous, divisive and, ultimately, too inexperienced for the top job.
They think if he is on the ticket, he could drive votes to Democrats in other key elections, as well as keep them in the White House. They had been expecting a moment like this. Some had been hoping for it.
On social media on Friday morning, candidates, party workers, strategists were piling into Trump.
For many other candidates, this would have been the beginning of the end. But this is no ordinary campaign. And Trump is no ordinary candidate.
Source: Al Jazeera