US Democratic presidential front-runner Hillary Clinton came under early criticism in a debate on Saturday for her 2003 vote backing the US invasion of Iraq, with Bernie Sanders linking it to regional chaos that aided the rise of Islamic State militants.The day after a series of deadly bomb and gun attacks that killed at least 129 people in Paris, Sanders said the invasion was "one of the worst foreign policy blunders in the modern history of the United States.""I would argue that the disastrous invasion of Iraq, something that I strongly opposed, unraveled the region immensely," said Sanders. He said it led to the rise of al-Qaida and the Islamic State.Clinton, who has frequently said the vote was 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 realize that it has antecedents to what happened in Iraq," she said.The exchange came early in the second debate for Democrats seeking their party's nomination for the November 2016 presidential election, with the focus shifted to foreign policy and ways to combat terrorism after the Paris attacks.The candidates and audience at Drake University in Des Moines, Iowa, observed a moment of silence at the beginning of the debate to honor those killed in France.Clinton, the former secretary of state, has far more foreign policy experience than either of her rivals - Sanders a US senator from Vermont and former Maryland Governor Martin O'Malley.But she was forced on the defensive early about her years of leading President Barack Obama's foreign policy team.Republican presidential contenders have consistently criticized Obama for what they say was an inadequate response to the rise of Islamic State in Iraq and Syria. In the aftermath of the attacks, businessman Donald Trump and US Senator Ted Cruz called on the administration to reconsider plans to allow thousands of Syrian refugees to be resettled in the United States.The White House aims to increase to 10,000 the number of Syrian refugees accepted in the United States during fiscal year 2016, from less than 2,000 accepted in the previous year.Clinton has always been from the more hawkish wing of the Democratic Party. Her support for the 2003 Iraq invasion played a major role in her primary loss to Obama in the 2008 White House race.