The Democratic presidential hopefuls have clashed over how to deal with militant group Islamic State, in the wake of deadly terror attacks in Paris.
Hillary Clinton was challenged by rival Bernie Sanders at a televised debate for backing the Iraq War, which he says led to the rise of the militants.
She said it was up to others to lead the fight against IS and called on Turkey and the Gulf states to do more.
The attacks killed 129 people and injured hundreds in the French capital.
Hours after the near-simultaneous attacks, CBS News said the debate in Iowa would re-adjust its focus to put more emphasis on counter-terrorism and foreign policy.
A moment’s silence was observed in Des Moines before the debate began, and the three candidates expressed their condolences with France.
But then they clashed over how best to deal with IS, which has claimed responsibility for the atrocities.
Mrs Clinton, the former secretary of state, said: “It cannot be an American fight… we will support those who will take the fight to Isis.”
But former Maryland Governor Martin O’Malley disagreed, saying: “America is best when standing up to evil in this world.”
And Mr Sanders, a Vermont senator, also attacked the former secretary of state Mrs Clinton when she said the US did not have the “bulk of responsibility” for the instability in the region.
The invasion of Iraq, which she backed, was to blame for the rise of al-Qaeda and IS, he said.
In other highlights:
- Mr O’Malley attacked “immigrant bashing” Republican Donald Trump’s plan to build a wall to keep Mexicans out
- he and Mr Sanders stood by their calls on the US to accept 65,000 Syrian refugees
- Mrs Clinton called on Turkey and the Gulf states to “make up their minds” about fighting jihadists
- she backed a minimum wage of $12/hour, but Mr Sanders wants $15/hour
- he criticised her for having so much Wall Street money backing her
- Mr Sanders said he will make public college tuition free and will pay for it by raising taxes
This primetime showdown is the party’s second debate of the election campaign, two fewer than the Republicans, who have a much wider field.
In 79 days, Iowa will be the first state to pick a presidential candidate from each party.
Voters across the US will go to the polls finally in November 2016 to choose the new occupant of the White House.