Donald Trump just got roughed up, and badly, by a girl. On Monday night, at the first presidential debate, Hillary Clinton made her opponent look ignorant, unprepared, egotistical, childish, petulant, impatient and at times totally incoherent.

How bad did it get? At one point, as Trump was groping blindly across the minefields of foreign policy, losing a foot here and a leg there, he announced, apropos of nothing, that “I think my strongest asset, maybe by far, is my temperament.” Clinton smiled sweetly and exclaimed, “Whew, okay!” The audience at Hofstra University, sternly instructed to remain silent throughout the debate, ignored the rules and burst into laughter.

They were laughing at you, Donald, not with you.

Clinton then patiently explained the importance of honoring international agreements, such as the NATO treaty, to a man who seemed not to grasp the concept of the nation’s word being its bond. One hopes her reassurances were enough to coax allies in Berlin, Tokyo, Seoul and other capitals down from the ceiling.

Here are the key moments from the first 2016 presidential debate between Hillary Clinton and Donald Trump on Sept. 26. NBC Nightly News anchor Lester Holt moderated the debate at Hofstra University in New York. (Sarah Parnass/The Washington Post)

[Dana Milbank: What worked for Trump in the primaries failed him against Clinton]

The 90-minute encounter, moderated by NBC’s Lester Holt, was less a debate than a beat-down. Clinton obviously had put in many hours of preparation. Trump apparently decided to wing it — and while this approach worked well during the Republican primaries, when nobody got much time to speak and pithy one-liners could win the day, it bombed in a one-on-one clash where there was no place to hide.

Trump’s biggest vulnerability is that he so rarely knows what he’s talking about. Minutes before his hilarious temperament declaration, he had been boasting that his criticism of NATO a few months ago caused the alliance to begin focusing on terrorism. “I think we have to get NATO to go into the Middle East with us, in addition to surrounding nations,” he said.

Clinton coolly reminded him — “informed him” would probably be more accurate — of some pertinent facts. “You know, NATO as a military alliance has something called Article 5, and basically it says this: An attack on one is an attack on all,” she said. “And you know the only time it’s ever been invoked? After 9/11, when the 28 nations of NATO said that they would go to Afghanistan with us to fight terrorism, something that they still are doing by our side.”

That’s pretty much the way the evening went, especially toward the end. Trump visibly ran out of gas, poor thing. His answers became increasingly scattered and elliptical. Pressed to defend his contention (long since disproved) that he was against the Iraq War, he complained repeatedly that “everybody refuses to call Sean Hannity.” Even Hannity, the Fox News host, must have been perplexed.

This was after he charged that Clinton “doesn’t have the stamina” to be president. But she looked fresh as a daisy throughout, while Trump wilted before our eyes.

One of Trump’s worst moments, at least to my eyes and ears, came when Clinton alleged that he paid no federal income taxes in at least some recent years. Trump offered no protest, instead interjecting, “That makes me smart.” Seriously? No one wants to pay more in taxes than required, but the idea of a self-proclaimed billionaire getting a free pass will be hard for many voters to swallow.

[E.J. Dionne: Clinton shifts the election in her direction]

Throughout the debate, the split screen showed Trump mugging, fidgeting, shrugging, grimacing, offering an array of exaggerated smiles and frowns. He interrupted Clinton frequently, but she didn’t complain. She may have calculated that it benefited her cause for Trump to have the floor.

Clinton also may have tried to bait Trump by suggesting he succeeded in business only because of his father’s help — and might not be as wealthy as he claims. If so, the tactic worked. Trump was unable to let anything pass. For reasons I can’t begin to understand, he even renewed his long-running feud with comedian Rosie O’Donnell.

I am under no illusion that Trump’s abysmal performance will cause his most dedicated supporters to have second thoughts. They heard his central argument, which is that “politicians like Secretary Clinton” have failed — and it’s time to try something new.

But while the race has tightened to the point where Trump could actually win, Monday night vividly demonstrated why he should not — why he must not. Whether you like Clinton or not, it’s obvious that she can do the job. The debate had to make undecided voters question whether Trump even has a clue.

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