Going into the debate, it appeared that former Secretary of State Hillary Clinton, Vermont Sen. Bernie Sanders and former Maryland Gov. Martin O'Malley would focus largely on domestic issues, a reflection of the topics that dominate their campaigning across the country. The tone of the debate also appeared likely to be combative, with both O'Malley and Sanders preparing to go after Clinton if needed.
That all changed on Friday night when the attack, for which ISIS has claimed responsibility, left at least 129 people dead and hundreds of others wounded in the streets of France's capital city.
A large portion of the debate will now certainly focus on combatting terrorism, on each candidate's foreign policy and on their national security credentials, comfortable ground for Clinton, who once served as America's top diplomat. Sanders and O'Malley will now have to show they can be commander in chief and handle challenges on the international stage, too.
Here are six lines of questioning that could come up on Saturday night and how the candidates would likely deal with them:
For Clinton: You were secretary of state for four years during a time that saw a rise of instability in the Arab world that paved the way to extremism like ISIS. How can voters trust that you, as president, can effectively combat terrorism?
Because of her four years as secretary, Clinton is by far the most prepared candidate on the stage to discuss foreign policy issues. That is likely to be a key point she stresses to make the case that she is best positioned to become commander in chief on day one. But her experience cuts both ways.
She has a record of dealing with terrorism, one that can be questioned and critiqued. And her positions on national security put her to the right of her former boss, both men on the stage Saturday night and many of the most animated Democratic primary voters.
Clinton was asked this week in New Hampshire about the lessons she learned from her vote to authorize force in Iraq in 2002 as a senator. As she does regularly, Clinton said that vote was a "mistake." But this time she went further, arguing that her vote makes her more skeptical of using force again.
"I believe that we are now having to take a very hard look at making sure that force is always used only as a last resort, not a first choice," Clinton said.
For Sanders and O'Malley: Neither of you have ever operated on the international stage and much of your presidential platforms focus on domestic issues. How can voters trust that you are prepared, as president, to effectively combat terrorism?
A debate on foreign policy and terrorism is not comfortable ground for Sanders -- whose stump speech is chock-full of domestic policy.
Sanders voted against the Iraq War in 2002 and has said on the campaign trail that he would defend the nation "responsibly." But Saturday's debate will force the candidate to expand on those ideas.
O'Malley, on the other hand, will likely fall back on his executive experience as the reason he could handle terrorism. At a speech in June on foreign policy -- his first and possibly last of the campaign specifically on the topic -- O'Malley highlighted the need for "human intelligence" to combat ISIS and cautioned against a "mindless rush to war," like he said happened with Iraq in 2002.
Doug Wilson, a top O'Malley policy adviser, defended O'Malley's chops on foreign policy after the speech, noting that "the governor has traveled" and "met with foreign leaders."
Boots on the ground
Now that ISIS has claimed it struck Western soil, are you willing to commit a serious amount of U.S. military boots on the ground in Syria and Iraq as part of an international effort to destroy the group?
President Barack Obama has already authorized the use of up to 50 Special Operations forces in Syria to combat ISIS, in addition to mor than 3,500 in Iraq.
But as French President Francois Hollande promises a "very determined" response, there are certain to be renewed calls to use U.S. military force more broadly and directly against ISIS -- particularly if France ups its presence in the Middle East.
In a statement after the Paris attacks, Clinton suggested she wouldn't break from France.
"We must stand side-by-side every step of the way with France and our allies around the world to wage and win the struggle against terrorism and violent extremism," she said.
Clinton has been more hawkish than her Democratic primary rivals. As secretary of state, she pushed for a larger U.S. role in Syria, whose civil war has allowed ISIS to seize swaths of territory. But during the campaign so far, she has opposed a larger U.S. role in a ground war in the region.
Sanders, meanwhile, strongly opposed putting U.S. boots on the ground in the first Democratic debate.
"I will do everything that I can to make sure that the United States does not get involved in another quagmire like we did in Iraq, the worst foreign policy blunder in the history of this country," he said then, in a veiled shot at Clinton, who voted to go to war there.
"We should be putting together a coalition of Arab countries who should be leading the effort," Sanders said. "We should be supportive, but I do not support American ground troops in Syria."
With years of hindsight, our strategy in Syria is not producing the results anyone would have wanted. The moderate opposition that the U.S. backs has not broken President Bashar al-Assad's grip on power, and the refugee crisis has continued to spiral out of control. Would you get more involved as president?
Despite the candidates' shared opposition to putting American troops on the ground, that doesn't mean involvement in the conflict can't be escalated: Ground troops are just one aspect of a Syria strategy.
Clinton, for example, backed providing more assistance to the Syrian rebels as secretary of state and advocated a no-fly zone to help protect Syrian refugees and end the carnage.
Both Sanders and O'Malley have opposed a no-fly zone, using it, at times, to knock Clinton as hawkish.
Sanders said he would oppose the U.S. getting "more deeply involved in that horrible civil war," worrying that a no-fly zone would "lead to a never-ending U.S. entanglement in that region."
O'Malley has cast the possible tactic as more difficult because of Russian activity in the country.
"You have to enforce no-fly zones, and I believe, especially with the Russian air force in the air, it could lead to an escalation because of an accident that we would deeply regret," he said at the previous debate.
As president, would you be willing to work with Russia and President Vladimir Putin to combat ISIS in Syria and Iraq?
Russia is conducting military strikes in Syria and any sizable U.S. operation in the country would force the United States to, at some point, coordinate with the country. But many see Putin's presence there as largely aimed at supporting the Assad regime -- which the U.S. opposes -- and to challenge American power in the region. Not to mention that he has a habit of belittling U.S. leadership.
At the first debate in Las Vegas, Clinton said the United States needed to "stand up to (Putin's) bullying, and specifically in Syria, it is important."
But she has also expressed openness to working with Russia when touting a no-fly zone, saying in Iowa in October that "the Russians would have to be part of (a no-fly zone) or it wouldn't work."
How far does that extend, though? Would Clinton, as president, agree to work with the Russians in operations to defeat ISIS? Or would she be inclined, as Obama has said, to hold back and let Russia sink in the "quagmire" that he predicted awaited Putin in his involvement in the war-torn region.
Sanders and O'Malley haven't been clear about whether they would work with Russia more directly as president.
At the debate, Sanders said that, "Mr. Putin is going to regret what he is doing" in Syria, but did not address why and how he would deal with him. Sanders backed sanctioning Russia during its incursion into Ukraine, but his reticence to get more deeply involved in Syria makes it unlikely that he would ever work with Putin on combating ISIS.
O'Malley, on the other hand, does not regularly address Putin or Russia. During his July speech on foreign policy, for example, the presidential candidate didn't mention the Russian leader.
In light of the terrorist attacks in Paris, which some are already labeling a major intelligence failure, does the United States need to bolster its domestic surveillance programs?
Clinton and Sanders clashed during the first debate on National Security Agency spying -- a topic that's likely to return with new weight, given French intelligence agencies' inability to prevent Friday's attacks.
Like Iraq, their differences on the issue date back more than a decade -- to Clinton's vote soon after September 11 for the Patriot Act, which broadened the U.S. government's abilities to gather intelligence on suspicious individuals on American soil.
In the first debate, Sanders said he would halt the NSA's surveillance efforts. "I'd shut down what exists right now, (which) is that virtually every telephone call in this country ends up in a file at the NSA," he said. "That is unacceptable to me."
Clinton, for her part, accused George W. Bush's administration of stretching its surveillance powers too far but said it was "necessary to make sure that we were able after 9/11 to put in place the security that was needed."
The two also disagreed at the debate over Edward Snowden, the NSA leaker. Both said he broke the law, but Sanders said the benefit of his leaks in making Americans aware of what the government was doing should be taken into consideration.
Were you wrong to call for the United States to admit more Syrian refugees?
Republican presidential candidates hammered this point in the hours before the debate. They've criticized the Obama administration's plan to allow 10,000 Syrian refugees into the country in 2016 -- and have tied that criticism to the Democratic presidential field, which has called for even larger numbers.
And their arguments were strengthened by France's decision to close its borders following the attacks.
"President Obama and Hillary Clinton's idea that we should bring tens of thousands of Syrian Muslim refugees to America, it is nothing less than lunacy," Texas Sen. Ted Cruz said on Fox News, in one example of the criticism.
Sanders has resisted pinning a specific number on the Syrian refugees accepted by the United States. But O'Malley led the charge for the U.S. to admit more Syrian refugees, calling months ago for the United States to accept 65,000 next year.
Clinton agreed, matching O'Malley's figure of 65,000 in an appearance on CBS' "Face the Nation."
"I would like to see us move from what is a good start with 10,000, to 65,000, and begin immediately to put into place the mechanisms for vetting the people that we would take in, looking to really emphasize some of those who are most vulnerable," Clinton said in that appearance.
The three Democrats are likely to be pressed on whether they stand by those calls to aid refugees of Syria's civil war and welcome them into the United States.