Donald TrumpDonald John TrumpPompeo changes staff for Russia meeting after concerns raised about top negotiator's ties: report House unravels with rise of 'Les Enfants Terrible' Ben Carson: Trump is not a racist and his comments were not racist 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 ObamaImmigrant advocacy groups shouldn't be opposing Trump's raids More adult Twitter users follow Obama than Trump: survey Trump's no racist; he's an equal opportunity offender 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 CruzGoogle official denies allegations of ties to China The Hill's Morning Report - Presented by JUUL Labs - House to vote to condemn Trump tweet Cruz in 2016 said 'something fundamentally wrong' with Christians who back Trump: book MORE], who's also running for president," Dickerson said.

Katie Williams contributed.