It’s hard to pick the worst moment for Donald Trump on a night during which he flailed trying to find balance. But an early gaffe largely lost in the cross-talk indicated that he was more easily baited, and thirstier for blood, than he was prepared for a presidential debate.

And it smacked of a mistake that helped cost Gerald Ford a return engagement as president in 1976.

“You’re telling the enemy everything you want to do,” Trump said in a familiar criticism of Clinton’s plan for dealing with the Islamic State. Then he deviated: “No wonder you’ve been fighting — no wonder you’ve been fighting ISIS your entire adult life.”

Back in 1976, Ford said “there is no Soviet domination of Eastern Europe,” which, of course there was. Trump placed the birth of ISIS in 1965, about five decades too early.

It’s not that Trump really thinks ISIS formed when Clinton was 18, or that she’s only been an adult for the last several years. But Clinton was under his skin, and he reacted like a child by simply blurting out the first attack that came to mind.

[Five Objectives for Hillary Clinton in the Debates]

Clinton called for fact-checkers to take a look at that one as the candidates and moderator Lester Holt grappled for control of the debate. In any normal year, such a wild claim might be disqualifying — it displayed a lack of knowledge of ISIS far worse than Rick Perry’s inability to name a Cabinet department on command. But in a year when facts don’t seem to matter, the truth was far less important than what the response said about Trump’s temperament.

He couldn’t keep his cool in a pressure-packed moment. Clinton had just countered a Trump attack on her ISIS proposal by taunting him. “At least I have a plan,” she said.

Then, Trump lost his composure. In doing so, he underscored the marquee phrase of her Democratic convention speech, when she said “a man you can bait with a Tweet is not a man we can trust with nuclear weapons.”

After that, Trump never recovered for long. And that must have been very frustrating for his fans — especially after he effectively crushed Clinton on trade in the debate’s opening moments.

But the ISIS exchange set the table for Clinton to calmly prosecute her case against Trump on taxes, foreign policy and criminal justice. She was so much better on substance that Trump quietly agreed with her at least three times.

[Hillary’s Honesty and Trump’s Temperament]

Holt countered Trump when he lied about his pre-war support for the Iraq invasion and his role in promoting what Clinton called the “racist birther lie.”

Trump and his spinners can insist he won, but they know in their hearts that Clinton, who wasn’t even at her best, wiped the floor with him. Trump’s ISIS gaffe was that of a bush-leaguer stepping to the plate at Yankee Stadium and buckling at the sight of his first big-league curve.

Indeed, Clinton’s missteps might have stood out more against a more seasoned debater who was positioned to take advantage of them rather than turning focus back on his own shortcomings. But in the end, Trump lied, denied and bullied his way to the worst debate performance since the advent of television.

Roll Call columnist Jonathan Allen is co-author of the New York Times-bestselling Clinton biography “HRC” and has covered Congress, the White House and elections over the past 15 years.