Here's a quick rundown:
* The sixth — you read that right — GOP debate saw three fault lines open widely: businessman Donald Trump vs. Texas Sen. Ted Cruz, most notably; New Jersey Gov. Chris Christie vs. Florida Sen. Marco Rubio; and Rubio vs. Cruz.
* After a long period where they avoided criticizing each other, Cruz and Trump entered into open war. It started when Trump recently began questioning whether Cruz was eligible to be president, because he was born to a Cuban father and American mother in Canada.
* Christie and Rubio also mixed it up over who, politically, was the most conservative candidate. And Rubio attacked Cruz over his immigration policies.
* Cruz seemed to put on the most solid performance of the evening, parrying attacks well and landing one-liners. Former Florida Gov. Jeb Bush continued to struggle to break through.
* The debate was important because it was the second-to-last debate before the Iowa caucuses, the first official vote of the nominating season.
* Trump and Cruz also sparred over Cruz accusing the businessman of having "New York values" — essentially trying to paint him as a city slicker. Trump responded with a long answer about 9/11.
* Sen. Rand Paul, who has low polling numbers, refused to participate in the debate. At one point protesters in the audience stood up and yelled, "We want Rand!"
Round 1: Cruz vs. Trump
Cruz attacked Trump for raising birther questions: "Donald's mother was born in Scotland, she was naturalized."
Cruz went hard against Trump in South Carolina, criticizing the front-runner for questioning Cruz's legal ability to run for president — drawing boos and cheers for both candidates.
"I recognize that Donald is dismayed that his poll numbers are falling," Cruz said. "But the facts and the law are quite clear. The child of a citizen born abroad is a natural U.S. citizen."
In recent weeks, Trump, other Republicans, and some constitutional scholars have questioned whether Cruz, who was born in Canada of an American mother and Cuban father, is eligible to run for the highest office in the United States.
Cruz said that under "birther" theories — which he said would require candidates for president to be born in U.S. soil of two American parents — would not only disqualify him, but also Marco Rubio, Bobby Jindal, and Donald Trump, whose mother was born in Scotland.
"Donald's mother was born in Scotland, she was naturalized," Cruz said.
"On the issue of citizenship, I'm not going to use your mother's birth against you," Cruz said.
Trump responded initially by saying he "didn't care" whether Cruz was a natural-born citizen or not because he was certain he was going to win. Immediately afterwards, however, Trump said that "very, very fine constitutional attorneys" feel that Cruz is not eligible to run.
"There's a big question mark on your head," Trump said. "And you can't do that to the party."
Read more here.
Round 2: Rubio vs. Cruz
Rubio and Cruz had a heated exchange on immigration, in which they accused each other of supporting legalization efforts and of changing their positions only when it became politically expedient.
Asked whether his thinking had changed since he supported changes to the nation's immigration laws two years ago, Rubio said the emergence of ISIS had transformed the issue into a national security problem.
"The issue is dramatically different from what it was 24 months ago," Rubio said. "24 months ago you didn't have a group of radical crazies named ISIS who are burning people in cages and recruiting people to enter our country legally. They have a sophisticated understanding of our legal immigration system, and we now have an obligation to ensure that they are not able to use that system against us."
Cruz then interjected to rebut Rubio, saying that he had always rejected softening immigration restrictions out of a concern for national security.
"Radical Islamic terrorism wasn't invented 24 months ago," Cruz said, adding that he led the way in stopping an amnesty bill, "because it was clear then as it is clear now that border security is national security."
Rubio fired back by criticizing Cruz for dramatically changing his position on immigration.
"You used to say you supported doubling the number of Green Cards," Rubio said. "Now you say you are against it. You used to support a 500% increase in guest workers. Now you say you are against it. You used to support legalizing people who were here illegally."
Cruz said Rubio's "opposition research" was false and insisted that Rubio had supported legalization for 12 million "illegals," drawing boos from the crowd.
The two Cuban-American candidates have battled for months over immigration, with Rubio accusing Cruz of downplaying his past support for comprehensive immigration reform in favor of a more hardline stance.
The policy fight is byzantine, drawing on specific moments from the recent failed effort to pass an immigration bill through the Republican Congress that was led in part by Rubio. The Florida senator distanced himself from the bill after it passed with bipartisan support in the Senate but failed in the House.
Round 3: Christie vs. Rubio
Christie and Rubio are both vying for the establishment slot in the Trump-Cruz dominated primary.
The central themes: Rubio accused Christie of being a liberal, and Christie accused Rubio of being a lightweight. The snapping back and forth got nasty, reflecting how much stake each man sees in taking down the other.
"I like Chris Christie, but we can not afford to have a president of the United States that supports Common Core," Rubio said during one exchange, regarding education policy. In other exchanges, he accused Christie of supporting gun control and said he "wrote a check to Planned Parenthood."
The subtext: Christie is too far left for the Republican primary.
"Unfortunately, Governor Christie has endorsed many of the ideas that Barack Obama supports," Rubio said at one point.
Christie said Rubio had misconstrued his record — and said that Rubio didn't have that much substance of his own to offer.
"Listen, this is the difference between being a governor and a senator," Christie said. "See when you're a senator, what you get to do is just talk and talk and talk. And you talk so much that nobody can ever keep up with what you're saying is accurate or not."
And now, some notable quotes and one-liners of the evening:
* "I just left Indonesia — bomb, bomb, bomb, bomb." —Trump (BONUS VINE BELOW)
* " That same columnist wrote a column comparing me to an evil demonic spirit from the movie, It Follows, that jumps apparently from body to body possessing people." —Cruz on this New York Times piece about campaign loans.
* "I know Bernie and I can promise you he's not going to be President of the United States." —Kasich
* "Is this America anymore?" —Carson, when asked about Clintons and veering into internet comments.
* "We are not going to have peace. We are not going to have peace in Syira. The Syrians should stay in Syria." —Christie
* "We want Rand! We want Rand! We want Rand!" —Protesters in the audience. (Rand Paul didn't make the cut for this debate.)
* "Donald's mother was born in Scotland, she was naturalized." —Cruz attacking Trump for his birther comments.
* "No." —Trump on whether he is reconsidering his proposed ban on Muslims.
* "Hillary Rodham Clinton is disqualified from being Commander-in-Chief of the United States." —Rubio on Clinton's email controversy.
* "It's the New York Times. They're always wrong." —Trump on statements in the Times that he will raise tariffs 45% on Chinese goods. (The Times responded to that here).
* "Iranian wiseguys." —Trump, presented without context.
Trump had his strongest moment of the debate Thursday when he took on Ted Cruz's attack on his "New York values." People had their own interpretation of what that means — and many others searched to find out:
Cruz made the comment earlier this week, saying Trump should play the song "New York, New York" at his rallies because he "embodies New York values." At the time, Cruz did not elaborate on what they were.
At the debate, moderator Maria Bartiromo asked Cruz to explain, noting that she, too, was from New York City. Cruz said South Carolinians know what "New York values" means.
"I think most people know exactly what New York values are," he said.
"And listen, there are many, many wonderful, wonderful working men and women in the state of New York. But everyone understands that the values in New York City are socially liberal or pro-abortion or pro- gay-marriage, focus around money and the media," Cruz said.
"And -- and I guess I can -- can frame it another way. Not a lot of conservatives come out of Manhattan. I'm just saying," Cruz said.
Trump shot back: "So conservatives actually do come out of Manhattan, including William F. Buckley and others, just so you understand."
Given the floor by Bartiromo, Trump defended his city, speaking at length about how New Yorkers dealt with 9/11 and earning applause from the audience.
Here it is in full:
And just so -- if I could, because he insulted a lot of people. I've had more calls on that statement that Ted made -- New York is a great place. It's got great people, it's got loving people, wonderful people.
When the World Trade Center came down, I saw something that no place on Earth could have handled more beautifully, more humanely than New York. You had two 100 -- you had two 110-story buildings come crashing down. I saw them come down. Thousands of people killed, and the cleanup started the next day, and it was the most horrific cleanup, probably in the history of doing this, and in construction. I was down there, and I've never seen anything like it.
And the people in New York fought and fought and fought, and we saw more death, and even the smell of death -- nobody understood it. And it was with us for months, the smell, the air. And we rebuilt downtown Manhattan, and everybody in the world watched and everybody in the world loved New York and loved New Yorkers. And I have to tell you, that was a very insulting statement that Ted made.
Bush struck out at Hillary Clinton with this comment referencing her using a personal email account while serving as Secretary of State. Here's the best line:
Cruz said the big problem with the New York Times report on Goldman Sachs and Citibank loans he failed to disclose during his Senate campaign is the New York Times.
The smaller problem is a Cruz mistake, Cruz said.
"I made a paperwork error," Cruz said. "If that's the best the New York Times has got, they better go back to the well."
The paper reported Wednesday that Cruz had failed to disclose loans from Citibank and Goldman Sachs on his FEC reports following his successful run for Senate from Texas. The loans were disclosed on Senate financial reports. On the trail Wednesday, Cruz told reporters his Senate campaign had made a mistake on the FEC filings and his team was working to correct the error.
On the debate stage Thursday, Cruz said the New York Times was an enemy of Cruz' political career.
"The New York Times and I don't exactly have the warmest of relationships," Cruz said, citing opinion pieces in the paper that had attacked him.
Trump said some Syrians coming to the U.S. could be a “Trojan horse” for terrorism, particularly given that so many Syrian immigrants on TV are men.
Trump, who supports banning Muslims from entering the country, was responding to a question about refugees — including Dr. Refaai Hamo, a scientist who fled Syria's civil war and was Obama's guest at the State of the Union on Tuesday. "When he introduced the doctor," Trump said of the president's address, "that is not what is representative of that line of immigration."
"It's not fear and terror, it's reality," Trump added, citing Thursday's terror attack on Thursday in Indonesia. Trump then pointed to last fall's terror attack in Paris, where gun control is stricter than the U.S., and cited the Muslim couple behind the terror attack last fall in San Bernardino, California. "That's what we have," Trump said.
Among Syrian refugees, Trump went on, you see "very few women, very few children." Instead, Americans see immigrants who are "strong powerful men. People look at that and wonder what is going on … We can't let these people come into our country and break our borders."
The moderators asked the candidates what they thought about Trump’s proposal that no Muslim immigrants should be admitted to the U.S. Here’s what they said:
* TRUMP: Still supports banning all Muslims from immigrating to the United States — but "only temporarily."
* BUSH: Believes all Muslims would alienate America's Arab allies, making forming a coalition against ISIS almost impossible.
* KASICH: Wants to "pause admitting the Syrian refugees."
* CHRISTIE: Would not take "any Syrian refugees of any kind."
* RUBIO: "If we do not know who you are and we don't know why you are coming, you are not getting into the U.S."
* CRUZ: Would not admit any refugees from nations that have a "big ISIS or al-Qaeda presence."
* CARSON: Would encourage displaced Syrians to resettle in a "safe" province of their own country.
And, as expected, most of the candidates hit President Obama’s tenure as commander-in-chief.
Asked about the positive economic numbers touted by Obama, Cruz spoke at length about the 10 American sailors who were held overnight by the Iranian military after their ships accidentally entered Iranian waters earlier this week. The Texas senator said that when the U.S. has a Republican president, Americans will not see their service people "on their knees."
Bush extended the critique to Clinton, saying that the FBI investigation on the Democratic candidate's use of email would mean that she would have to spend the beginning of her administration going back and forth between the "White House and the courthouse."
For his part, Carson echoed Santorum's comments in the "undercard" debate, when the former senator spoke at length about the "existential threat" posed by "electromagnetic pulses" generated by nuclear bombs in the "exo atmosphere." He also expressed concern about dirty bombs and cyber attacks.
Trump reiterated his concern about Syrian refugees, saying that most of the people fleeing ISIS are "strong powerful men" who could be "trojan horses" for the terrorist group, rather than women and children.
Let's end on a light note. Kasich is known for using very distinctive hand gestures while speaking. Which means this was inevitable:
Here's more coverage of the debate:
Here's some earlier coverage of the GOP race:
* Cruz: Loan Not In Campaign Disclosures Was "Technical And Inadvertent Filing Error"
* Can A Party Dominated By Trump Convince America It Cares About Poverty?
* Carly Fiorina, Unlike Rand Paul, Sees Undercard Debate As Opportunity
* Mike Huckabee: If Trump's The Nominee, "We Need To Unite Behind Him"
Reporting by Nicolas Medina Mora, Evan McMorris-Santoro, Dominic Holden, Austin Hunt, and Tom Namako