Bernie Sanders burned Hillary Clinton with his first opportunity at the debate

Sen. Bernie Sanders (I-Vermont) went after former Secretary of State Hillary Clinton from the very first question he received at Saturday night's Democratic debate.

Asked about his assertion that climate change is the No. 1 threat to national security, Sanders pivoted to a critique of Clinton's vote in favor of the Iraq war

He said that decision helped destabilize the Middle East, leading to the rise of ISIS.

"In fact, I would argue that the disastrous invasion of Iraq, something I strongly opposed, has unraveled the region completely has lead to the rise of Al Qaeda and ISIS," Sanders said.

"I don't think any sensible person would disagree that the invasion of Iraq led to the massive level of instability that we're seeing right now. I think that was one of the worst foreign policy blunders in the modern history of the United States."

Clinton refuted Sanders' criticism. She said that though the invasion of Iraq was destabilizing, radical extremism existed prior to the war.

"I have said that the invasion of Iraq is a mistake, but I think if we are really ever going to really tackle the problems posed by jihadi extreme terrorism, we need to understand it and realize that it has antecedents to what happened in Iraq," Clinton said.

Clinton has a history of fielding criticism of her vote in favor of the war in Iraq. It was a major roadblock for Clinton in the 2008 presidential race.