Moderator John Dickerson pointed out that Trump has said he would work with Russia as the country gets involved in Syria, ostensibly to fight the terrorist group ISIS.
Trump then negated Bush's stance on Russia not being an ally of the US, saying that "Jeb is so wrong" in his assessment.
Click through reactions to the debate here:
Bush retorted that Russian President Vladimir Putin wouldn't be an ally of the US and pointed out that Russian planes are attacking US allies in Syria. He called for a Sunni-led coalition on the ground to destroy ISIS.
"We're supporting troops that we don't even know who they are," Trump said, repeating the line. "We have no idea who they are."
Dickerson cut in and tried to move on, but Bush shot back at Trump.
"This is from a guy who gets his foreign policy from the shows," he said. "This is a guy who thinks that Hillary Clinton is a great negotiator in Iran. We're living in dangerous times. This is a man who insults his way to the nomination."
Trump started talking over him.
"He spent $24 million in New Hampshire and he's practically ... $24 million in New Hampshire," Trump said. "Give me a break!"
Dickerson cut in again.
"Gentlemen, let's leave it there so I can ask a question of Sen. Cruz, who is also running for president," he said, referring to Sen. Ted Cruz (R-Texas).
But it wasn't over there.
After Cruz answered a question, Bush and Trump went at it again. Dickerson asked Trump about former President George W. Bush campaigning for Jeb in South Carolina, mentioning that Trump has said that the former president should have been "impeached" for getting the US into the Iraq War.
"Obviously the war in Iraq was a big, fat mistake," Trump said. He then attacked Jeb Bush for waffling earlier in the campaign on whether the war was a mistake.
"It took him five days before his people told him what to say, and ultimately he said it was a mistake," Trump said.
Jeb then said that he was "sick and tired" of President Barack Obama blaming George W. Bush, his brother, for the "problems that he's had."
He continued: "I could care less about the insults that Donald Trump gives to me. It's blood sport for him, he enjoys it, and I'm glad he's happy about it. But I am sick and tired, I am sick and tired of him going after my family. My dad [former President George H. W. Bush] is the greatest man alive in my mind. While Donald Trump was building a reality TV show, my brother was building a security apparatus to keep us safe. And I'm proud of what he did."
"The World Trade Center came down during your brother's reign," Trump retorted. "Remember that."
The crowd booed loudly.
While Jeb was saying that Trump has "had the gall" to go after his mother, Trump talked over him.
"That's not keeping us safe," Trump said, continuing his point.
Jeb continued talking about his family, saying that his mother is the strongest woman he knows.
"She should be running," Trump replied.
More from Saturday night's debate:
More from Business Insider: