As the presidential race heats up, the Republican candidates gathered last night for their fifth debate, this time in Las Vegas.

After starting late because the candidates were busy getting their photos taken backstage, the two-hour debate got under way, focusing mainly on terrorism and security.

How the future president will deal with the increasing threat of terror groups such as Islamic State has been a key focus of the race so far, particularly in the wake of the San Bernardino shooting last month.

Media scrutiny has been aimed largely at frontrunner Donald Trump, who has been accused of fostering Islamophobia. Last week, he released a campaign statement calling for a "total and complete shutdown of Muslims entering the United States until our country's representatives can figure out what's going on".

Trump also cited research – later deemed by experts to have come from an unreliable source – claiming to have found that "there is great hatred towards Americans by large segments of the Muslim population". Over in Britain, he was ridiculed for referring to Daily Mail columnist Katie Hopkins, who has been unable to back up her claims that the UK has "lost control of vast swathes of our country" to Muslims, as a "respected columnist".

Trump's incendiary remarks have been widely denounced, including by a number of his fellow candidates. Jeb Bush branded Trump "unhinged" while Lindsey Graham accused him of being "downright dangerous".

Last night, Bush was asked to expand on his characterisation of Trump. "Donald, you know, is great at the one-liners. And he's a chaos candidate but he'd be a chaos president," Bush said.

"He would not be the commander in chief we need to keep our country safe."

Trump fired back, "Jeb doesn't really believe I'm unhinged. He said that very simply because he has failed in his campaign. It's been a total disaster. Nobody cares."

Bush later responded, "Donald, you're not going to be able to insult your way to the presidency... Leadership is not about attacking people and disparaging people. Leadership is about creating a serious strategy."

The two squabbled for much of the debate, and further ridicule was aimed at Trump, who tried to explain his recent assertion that the US – specifically, Bill Gates – should consider "closing up" the internet to help stop radicalisation.

"ISIS is recruiting through the Internet. ISIS is using the Internet better than we are using the Internet, and it was our idea. What I wanted to do is I wanted to get our brilliant people from Silicon Valley and other places and figure out a way that ISIS cannot do what they're doing," Trump said last night.

"You talk freedom of speech. You talk freedom of anything you want. I don't want them using our Internet to take our young, impressionable youth... we should be using our brilliant people, our most brilliant minds to figure a way that ISIS cannot use the Internet."

His plans were met with significant mockery on Twitter.

More seriously, however, Trump reiterated his December 2 statement that he would kill the families of ISIS militants.

"I would knock the hell out of ISIS, I would hit them so hard," he previously told Fox. "When you get these terrorists, you have to take out their families. They care about their lives, don't kid yourself. When they say they don't care about their lives, you have to take out their families."

Last night, Trump said, "We have to be much tougher. We have to be much stronger than we've been... I would be very, very firm with families. Frankly, that will make people think because they may not care much about their lives, but they do care, believe it or not, about their families' lives."

Bush branded Trump's comments "crazy" and "another example of the lack of seriousness" of his fellow candidate's campaign.

Rand Paul later also questioned whether Trump was a serious candidate. "If you are going to kill the families of terrorists, realise that there's something called the Geneva Convention we're going to have to pull out of," he said.

"It would defy every norm that is America. So when you ask yourself, whoever you are, that think you're going to support Donald Trump, think, do you believe in the Constitution?"

Another key topic covered during the debate was background checks for Syrian refugees trying to enter the US. Ted Cruz claimed that Obama has been given "blanket authority to admit refugees... without mandating any background checks whatsoever," which was quickly refuted.

Cruz also said that the San Bernardino shooting was an example of what happens when the "FBI can't vet" refugees coming into America. Commentators were quick to point out that neither of the perpetrators of the massacre were Syrian, or refugees. In fact, one of them was born in Chicago, and the other spent most of her life in Saudi Arabia, a US ally.

However, Cruz did at least admit that there are "millions of peaceful Muslims around the world" and the fight against terror is "not a war on a faith; it's a war on a political and theocratic ideology".

While much of the commentary from last night's debate has focused on clashes between Trump and Bush, Marco Rubio and Ted Cruz also sparred over immigration reform. Rubio, the son of Cuban immigrants, wants to modernise the legal immigration system, but Cruz accused him of "fighting to grant amnesty and not to secure the border".

Unlike previous debates, there was little mention of God or faith. So far in the election race, religion has been high on the agenda for Republican candidates, with each of them vocal about their own faith, and several suggesting that religious freedom is under attack in the US.

However, last night the candidates steered away from much talk about their beliefs, perhaps taking heed from their Democrat counterparts. Even Ben Carson, who has a picture of himself and Jesus hanging in his home, only shoehorned a mention of God into his closing statement.

Despite this, some commentators drew attention to an apparent disparity between the rhetoric of the Republican candidates and their Christian faith.

The next Republican debate is scheduled for January 14 next year.