The long-anticipated clash between Ted Cruz and Donald Trump, for months just a soft rumble in the distance as both candidates were content to play nice, thundered onto the debate stage Thursday night.

With the two rivals locked in a tight race for frontrunner status, it was inevitable that a months-long détente between the anti-establishment firebrands would quickly disentegrate at the sixth GOP debate — one of the last nationally televised events before the Iowa caucuses.

Cruz, who no longer has the luxury of laughing off Trump’s barbs after his Iowa lead eroded in the last week, hit the stage ready to fight, forcefully dismissing Trump’s questioning of whether or not he could legally assume the presidency because he was born in Canada.

“Back in September, my friend Donald had said he had had his lawyers look at this every which way and said there was no issue there,” said Cruz, whose mother was born in the U.S. “Now since September, the Constitution hasn’t changed. But the poll numbers have.”

Trump admitted that he only recently started to play the “birther” card against Cruz as the Texas senator has risen in the polls.

“Now he’s doing a little bit better. Before I didn’t care,” said Trump, who vowed not to sue Cruz himself over citizenship questions but predicted that Democrats would should he win the Republican nomination. “There’s a big question mark over your head, and you can’t do that to the party.”

The fireworks between the two had been a long time coming, following months in which Cruz was content to play the pilot fish to Trump’s Great White shark. With the Iowa caucuses just 18 days away and Cruz and Trump still battling for position atop the field in Iowa and beyond, it was almost inevitable.

Cruz, whose lead in Iowa has narrowed as Trump has injected these citizenship questions into the broader campaign bloodstream, also pushed back at a New York Times report this week that he failed to properly disclose a loan from Goldman Sachs during his 2012 senate race.

“You know, the New York Times and I don’t exactly have the warmest of relationships,” he said, ticking off and quoting from critical opinion columns. Cruz dismissed the omission as “a paperwork error.”

“If that’s the best hit the New York Times has got, they better go back to the well," Cruz said.

As the debate entered its second hour, there were more blows between Cruz and Trump when moderator Maria Bartiromo asked Cruz what he meant when he said Tuesday that Trump has “New York values.”

“Everyone understands the values in New York City are socially liberal, pro-abortion, pro-gay marriage and focus around money and the media,” said Cruz, who referred to an interview Trump did years ago with the late Tim Russert in which he explained his more moderate views as a product of his living in Manhattan.

“Let me put this a different way,” Cruz continued, readying a haymaker. “Not a lot of conservatives come out of Manhattan.”

The line, of course, was a riff on Trump’s oft-used line questioning Cruz’s evangelical Christianity that “not a lot of evangelicals come out of Cuba.”

Trump responded by holding up William F. Buckley as an example of an admirable New York conservative and by invoking the 9/11 attacks as evidence of his city’s grit and values.

“No place on earth could have handled that more beautifully, more humanely than New York,” Trump said. “That was a very insulting statement that Ted made.”

Trump also defended his proposal to temporarily ban Muslim immigration as a matter of national security from Bush, who asked Trump rather politely to “reconsider” the matter.

“I want security,” Trump said, again invoking 9/11. “We have to stop with political correctness. We have to get down to creating a country that’s not going to have the kind of problems that we’ve had with people flying planes into the World Trade Centers.”

“There’s something going on and it’s bad,” he continued.

Bush, who has found his footing of late as the Republican candidate most willing to criticize Trump on the trail, struggled to muster the same toughness with him on the stage. When Trump dismissed him as “weak” in the second hour of the debate, Bush grinned as he turned toward Trump.

“C’mon, man,” he said.

The opening minutes of the sixth GOP primary debate also featured a number of candidates, still vying for position in a fluid race, articulating a similarly dark view of an increasingly scary world and emphasizing their commander in chief credentials.

Cruz ducked the debate’s first question about the economy to talk about the 10 U.S. sailors temporarily detained by Iran. Jeb Bush mentioned a retired general supporting his campaign seated in the crowd. And Marco Rubio, eschewing the inspirational, uplifting message he’s carried for months and adopting a darker, more sobering tone, vowed to defeat ISIS and offered the strongest worded critique of the likely Democratic nominee.

“Hillary Clinton is disqualified from being commander in chief of the United States,” Rubio said. “Someone who cannot handle intelligence information appropriately cannot be commander in chief, and someone who lies to the families of those four victims in Benghazi can never be president of the United States.

“When I’m president of the United States we’re going to win this war against ISIS,” he continued, promising the group’s destruction and to give those captured alive “a one-way ticket to Guantanamo, Cuba, and we’re going to find out everything they know.”

Rubio and Christie, both battling for position in New Hampshire’s establishment lane, took aim at one another, with Rubio hitting Christie for his past support of Planned Parenthood and Christie pointing out the young Florida senator’s lack of executive experience.

“When you're senator, what you get to do is 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,” Christie said. “When you're a governor, you're held accountable for everything you do. And the people of New Jersey have seen it.

“I like Marco too. And, two years ago he called me a conservative reformer that New Jersey needed. That was before he was running against me. Now that he is he changed his tune.”

Rubio, who delivered stand-out debate performances last year, gasped for air time throughout much of the debate, trying at one point to wrench the spotlight away from the Trump-Cruz slugfest, quipping, “I hate to interrupt this episode of Court TV.” 

But Rubio closed strong, waiting until the debate's final moments to engage Cruz in a debate over their contrary positions on a value added tax, or VAT, and then blasting the Texas senator for changing his positions on immigration, an ethanol subsidy, TPP and whether Edward Snowden is a hero, which Rubio chalked up to “political calculation.”

Cruz snapped back by sarcastically thanking Rubio. "I appreciate you dumping your oppo research folder.”

“No, it’s your record,” Rubio responded.