Donald TrumpDonald John TrumpBiden slams Trump over golf gif hitting Clinton Trump Jr. declines further Secret Service protection: report Report: Mueller warned Manafort to expect an indictment MORE attacked the Bush family during the Republican presidential debate Saturday night, accusing the George W. Bush administration of lying to the public before going to war in Iraq.

"Obviously the war in Iraq was a big, fat mistake," Trump said, launching into a fierce critique of rival Jeb Bush's brother. 

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"George Bush made a mistake," the billionaire continued. "Obviously we can make mistakes, but that one was a beauty."

"We should have never been in Iraq," Trump added. "They lied, they said there were weapons of mass destruction. There were none and they knew that there were none."

Jeb Bush aggressively defended his brother.

"I'm sick and tired of Barack ObamaBarack Hussein ObamaGOP rep: North Korea wants Iran-type nuclear deal Dems fear lasting damage from Clinton-Sanders fight Iran's president warns US will pay 'high cost' if Trump ditches nuclear deal MORE blaming my brother for all the problems that he's had and frankly I could care less about the insults Donald Trump gives against me," the former Florida governor said. 

"I'm sick and tired of him going after my family. My dad is the greatest man alive in my mind," Bush continued, referencing former President George H.W. Bush.

Trump didn't relent, interjecting that the "World Trade Center came down" under his brother's watch, prompting a chorus of boos in the debate hall.

"This is just crazy. This is just nuts. Geez, oh man," Ohio Gov. John Kasich (R-Ohio), who spoke next, said of the exchange. 

George W. Bush is slated to make his first appearance on the campaign trail on behalf of Jeb Bush on Monday in South Carolina, which holds the next round of voting in the GOP race.

Bush and Trump repeatedly clashed during the debate. Before the slugfest on the Iraq War, Bush criticized Trump for supporting a partnership with Russia to take on terrorists in Syria, calling the idea "ludicrous."

"Jeb is so wrong, just so you understand," Trump said as boos erupted in the crowd.

"You know who that is, that's Jeb's special interests and lobbyists talking," Trump responded.

"You gotta fight ISIS first," he continued. "You can't fight two wars at one time. That's why we've been in the Middle East for 15 years and we haven't won anything. We've spent $5 trillion in the Middle East because of thinking like that."

He was repeatedly interrupted by loud booing, to which Trump responded, "I only tell the truth, lobbyists."

The exchange with Bush quickly devolved into a shouting match, with moderator John Dickerson ultimately forcing the candidates to drop the topic.

"Let's leave it there so I can ask a question of [Sen. Ted CruzRafael (Ted) Edward CruzSenate Dems hold floor talk-a-thon against latest ObamaCare repeal bill Overnight Finance: CBO to release limited analysis of ObamaCare repeal bill | DOJ investigates Equifax stock sales | House weighs tougher rules for banks dealing with North Korea GOP state lawmakers meet to plan possible constitutional convention MORE], who's also running for president," Dickerson said.

Katie Williams contributed.