WATCH: Donald Trump hammers George W. Bush for 9/11 during shoutfest with Jeb

GOP candidates and mounting rivals Donald Trump and Jeb Bush got locked in a tit-for-tat argument over foreign policy as the two took swipes at each other during Saturday night's debate in South Carolina.

The argument started over foreign policy and got heated when Trump attacked Bush's brother, former President George W. Bush, and his decision to invade Iraq in 2003.

"Obviously the war in Iraq was a big, fat mistake," Trump said. "It took Jeb Bush... It took him five days, he went back, it was a mistake, it wasn't a mistake, it took him five days before his people told him what to say and he ultimately said, it was a mistake... We should have never been in Iraq, we have destabilized the Middle East."

He then accused the Bush Administration of lying about the weapons of mass destruction in Iraq.

Jeb Bush responded by calling Trump's insults "blood sport."

"But I am sick and tired of him going after my family," Jeb Bush said. "And 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."

Trump then blamed George W. Bush for allowing the terrorist attacks on the World Trade Center in 20o1.

Watch the exchange, as posted to YouTube, here: