Hillary Clinton, the US Democratic presidential front-runner, has come under heavy criticism in a debate for her 2003 vote backing the invasion of Iraq.
A day after a series of bomb and gun attacks that killed at least 129 people in Paris, Bernie Sanders, Clinton's rival, criticised her vote authorising the Iraq invasion and said it was "one of the worst foreign policy blunders in the modern history of the United States".
Sanders also linked the invasion of Iraq to regional chaos that aided the rise of the Islamic State of Iraq and the Levant (ISIL) group.
"I'd argue that the disastrous invasion of Iraq, something that I strongly opposed, unravelled the region immensely, and led to the rise of al-Qaeda and to ISIL," Sanders said during Saturday's debate at Drake University in Des Moines, Iowa.
"I don't think any sensible person would disagree that the invasion of Iraq led to the massive level of instability we are seeing right now."
Clinton, who has frequently called the vote a mistake, said it should be placed in the historical context of years of terrorism before the invasion.
"If we're ever going to really tackle the problems posed by jihadi extremists, we need to understand it and realise that it has antecedents to what happened in Iraq," she said.
"This is an incredibly complicated region of the world.
"It's become more complicated. And many of the fights that are going on are not ones that the US has either started or have a role in."
Wall Street issue
The Iraq exchange came early in the second debate for Democrats seeking their party's nomination for the November 2016 presidential election, which focused on foreign policy and ways to "combat terrorism" after the Paris attacks.
Al Jazeera's Kimberly Halkett, reporting from Des Moines, said the Iraq invasion was something that Clinton has acknowledged in the past as a mistake and she repeated that throughout the debate.
"The other issue where Clinton came under attack was on the issue of Wall Street," she said.
"Sanders advocates reforming Wall Street and challenged Clinton on how she would she clean it up given that she has received millions in donations from members of Wall Street and Wall Street banks. Clinton, in reply, promised she had a comprehensive reform to do that."
Meanwhile, Clinton struck a sharp contrast to Obama's comments in an interview aired on Friday that ISIL had been contained.
"We have to look at ISIL as the leading threat of an international terror network. It cannot be contained, it must be defeated," she said.
The White House aims to increase the Syrian refugees intake to 10,000 nex year, up from less than 2,000 in the previous year.
All the Democratic candidates supported taking in some Syrian refugees, with Clinton emphasising the need for careful screening prior to the intake.
NOTES FROM CHRISTOPHER SHERIDAN, AL JAZEERA FIELD PRODUCER
A handful of Drake University students and Des Moines residents showed up outside the debate venue to show their support for the victims of Friday's Paris attacks. With signs reading "Iowans Stand with Paris" and "Drake Stands With Paris", some of them expressed their own fears that a similar attack could happen in the United States.
Ashley Ferguson, a Des Moines resident, hoped the candidates would raise the issue of how to counter any attack from ISIL on US soil. "It's concerning to me what's going to happen in the future and I want our candidates to talk about what we're going to do to protect our country," he said.
Meanwhile, at the debate, there was a newer, more aggressive Bernie Sanders. Not exactly the attack dog that Republican presidential candidate Donald Trump is, but when it came to the war in Iraq, he made it clear Hillary Clinton backed, in his words, one of the worst foreign-policy blunders in US history.
Later, one of Sanders' top advisers denied he was suggesting Clinton was to blame for ISIL, but went back to the theme of the Iraq War almost immediately.
Tad Devine, the adviser, said: "Bernie Sanders got it right, Hillary Clinton got it wrong."
Source: Al Jazeera and agencies