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In the aftermath of the Paris and San Bernardino attacks, Republican presidential candidates spar over national security in the fifth GOP debate, which aired on CNN.

FOX's Jared Halpern has more from Las Vegas:

Former Florida Governor Jeb Bush pointed to recent world events to highlight what he sees as not serious ideas from front-runner Donald Trump.

(Bush) "He's a chaos candidate and he'd be a chaos President. He would not be the Commander in Chief we need to keep our country safe."

But few others on stage engaged Trump, who did become irritated at the jabs.

(Trump) "He has failed in this campaign, it's been a total disaster, nobody cares."

Instead, some of the most heated exchanges were on serious policy disputes between first term Senators Marco Rubio and Ted Cruz...

(Rubio) "Anti-American dictators like Assad, who help Hezbollah, who help get those IEDs into Iraq... if they go... I will not shed a tear."

Cruz, calling for caution on regime change.

(Cruz) "We keep hearing from President Obama and Hillary Clinton and Washington Republicans that they're looking for these mythical moderate rebels. It's like purple unicorns, they never exist."

Rubio and Cruz also debating the limits to NSA surveillance. At one point Dr. Ben Carson asked who he'd side with... the retired neurosurgeon declined to do that.

In Las Vegas, Jared Halpern, FOX News.