DONALD Trump entered the presidential debate with all the momentum. He left it battered and bruised.

Trump spent his two hours on stage defending himself against relentless attacks from his rivals for the Republican nomination, Marco Rubio and Ted Cruz. He was also ruthlessly interrogated by Fox News moderators Megyn Kelly, Chris Wallace and Bret Baier. By the end of the debate, he looked tired and shaken.

On issue after issue, Trump was brutally exposed. These were the most important of them.

1. Trump University

Marco Rubio hammered Trump on the subject of his “university” — a self-help school he founded that former students are now suing, claiming it was a scam.

“We (Trump University) have a 98 per cent approval rating from the people who took the course. We have an “A” from the Better Business Bureau. And people like it,” Trump said in response. Rubio disputed those facts, and moderator Megyn Kelly stepped in with some of her own.

“With respect, we went back and looked at this. The rating from the Better Business Bureau was a D-minus. That’s the last publicly available rating, in 2010,” Kelly said.

“There was Trump University, which was a business that you started, and it was marketed to many people, and now there is a class-action of over 5,000 plaintiffs against you, Mr. Trump. And, it involves veterans, and it involves teachers, and it involves so-called little guys, working class, and lower-working class and middle class who say that they were fleeced, who say that it was a scam.

“The class has been certified, and in that case you counter-sued the lead plaintiff alleging that you were being defamed,” Kelly, a former lawyer, continued, before referring to the judgement in that counter-suit.

“The court threw out your counter-suit, and made you pay almost $800,000 dollars in legal fees of hers, and you made the same argument about 98 per cent of the people being happy with Trump University.

“This is what the Court of Appeals found. They said that the plaintiffs against you are like the (Bernie) Madoff victims ... They found that victims of con artists often sing the praises of their victimisers until they realise they have been fleeced.”

Trump was left to protest meekly: “Let’s see what happens in court.”

2. The budget

Trump has persistently claimed he will balance America’s federal budget — which is currently half a trillion dollars in deficit — by dismantling the Department of Education and Environmental Protection Agency, as well as cracking down on “waste, fraud and abuse”. When Wallace pressed Trump for more details, it didn’t go very well.

“Mr Trump, your numbers don’t add up,” the moderator said, before bringing said numbers up on screen.

“The total budget of the education department is $78 billion,” he said. “The entire budget for the Environmental Protection Agency is $8 billion. The deficit this year is $544 billion. That’s more than a half a trillion dollars. Your numbers don’t add up, sir.”

Trump responded by bringing up the US government’s “poor bidding procedures” with pharmaceutical companies. He said improving those procedures, and lowering the prices of drugs in the process, would save “hundreds of billions of dollars”. But again, Wallace checked his facts.

“You say that Medicare could save $300 billion a year negotiating lower drug prices. But Medicare only spends $78 billion a year on drugs,” he pointed out.

How do you cut $300 billion from a $78 billion program? Get the calculator back out, Donald.

3. Immigration

Trump has delighted in cultivating his reputation as a hardliner on illegal immigration, particularly when it comes to people crossing the US border with Mexico. He repeats the same mantra, over and over: “We’re going to build a wall, and Mexico’s going to pay for it.” On top of that, there’s his promise to deport every illegal immigrant currently in the country.

However, earlier this week, Buzzfeed revealed that the New York Times has an audio recording of Trump from an off-the-record chat. In it, the candidate supposedly admits his views on immigration are a lot more flexible than his supporters believe.

The Times will only release the audio with Trump’s permission, and Cruz repeatedly urged him to make it happen during the debate.

“If you didn’t tell them that, the tapes will prove you’re innocent,” Cruz said. “But if, in fact, you went to Manhattan and said I’m lying to the American people, then the voters have a right to know.”

Trump showed no signs he’s willing to authorise the tape’s release. Instead, he defended his “flexible” approach to policy.

“I have a very strong core. But I’ve never seen a successful person who wasn’t flexible, who didn’t have a certain degree of flexibility. You have to have a certain degree of flexibility,” Trump said.

That’s not a bad point, but Trump’s credibility on one of his core issues took a beating.

4. The military

Trump says he will order the US military to assassinate terrorists’ wives and children. If such a policy sounds extreme to you, that’s because it’s a war crime. The third moderator, Bret Baier, asked Trump what he would do if the military refused to follow his unlawful orders.

“Experts have said that when you ask the US military to carry out some of your campaign promises, specifically targeting terrorists’ families, and also the use of interrogation methods more extreme than waterboarding, the military will refuse because they’ve been trained to turn down and refuse illegal orders,” Baier said.

“So, what would you do as commander-in-chief if the US military refused to carry out those orders?”

“They won’t refuse,” Trump replied. “They’re not going to refuse me. Believe me.

“I’m a leader. I’ve always been a leader. I’ve never had any problem leading people. If I say do it, they’re going to do it. That’s what leadership is all about.”

Trump proceeded to argue in favour of waterboarding — and worse — to the sound of applause from his fans in the crowd.

“Can you imagine these people, these animals over in the Middle East, that chop off heads, sitting around talking and seeing that we’re having a hard problem with waterboarding? We should go for waterboarding and we should go tougher than waterboarding.”

Finally, pressed on his idea to target terrorists’ families, he doubled down.

“I have no problem with it,” Trump said. Social media reacted with disgust.

Trump defending war crimes: going after the families of terrorists. In a presidential debate. I wish this were a nightmare. — Megan McArdle (@asymmetricinfo) March 4, 2016

In past debates @realDonaldTrump has been disturbing. Tonight he's outright disgusting. — Michael Medved (@MedvedSHOW) March 4, 2016

5. Flip-flopping

It was Kelly’s turn to apply the screws again when the moderators brought up Trump’s frequent reversals of opinion. Fox News had prepared clips of Trump speaking about three different issues — and arguing for two diametrically opposed positions on each one.

“Mr Trump, one of the things people love about you is they believe you tell it like it is. But time and time again in this campaign, you have actually told the voters one thing only to reverse yourself within weeks or even sometimes days,” Kelly said.

“You have to be flexible, because you learn,” Trump responded.

“I really mean it. You have to show a degree of flexibility. If you’re going to be one way and you think it’s wrong, does that mean the rest of your life you have to go in the wrong direction because you don’t want to change?”

As the conversation came his way, Marco Rubio pounced.

“This isn’t just about flexibility. There’s a difference between flexibility and telling people whatever you think you need to say to get them to do what you want them to do,” he said.

“That’s what Donald has done throughout his career.”

6. Foreign policy

Rubio was the aggressor again when the debate turned to foreign policy. He portrayed Trump as ignorant and uninterested in world affairs.

“I think, as we’ve seen throughout this campaign, Donald has not shown a seriousness about the issues of foreign policy. He just simply hasn’t,” Rubio said.

“The next president of the United States is going to have eight years of a mess of a foreign policy to clean up. That’s why it can’t be Hillary Clinton. And quite frankly, that’s why it can’t be someone who simply has not shown the intellectual curiosity or the interest in learning about these very complicated issues. And Donald simply hasn’t.”

Instead of addressing specific policy details, Trump proved Rubio’s point, running back to his favourite subject — the polls.

“Well, let me just say this. I’ve gotten to know Marco over a period of time, believe me, he is not a leader. Believe me,” Trump said.

“I will prove to be a great leader. And, you know, it’s very interesting, we talk about the polls. Every single poll when it comes to IS and the military and the border say, by far, Trump is the best.”

That answer didn’t deter Rubio.

“Once again, he was pressed on a policy issue to show his understanding of foreign policy, and his reaction was just to attack somebody else with a name,” the Senator said.

“This is a time for seriousness on these issues. You have yet to answer a single serious question about any of this.”

It was a brutal exchange, like many others on that stage. But will Trump’s supporters care? We’ll soon find out.