"But this is a reminder, why we need to de-escalate tensions in the Middle East. The American people do not want a war with Iran," she warned.
Former Vice President Joe Biden, who in the days since the president authorized the drone strike against Qassem Soleimani -- the second most important official in Iran's government behind Ayatollah Khamenei -- has launched a torrent of criticism against Trump, said he would "hold off on commenting on the news until we know more."
"But there is one thing I will say, Jill and I are keeping our troops and Americans overseas in our prayers. We hope you’ll keep them in yours," he added.
Biden first learned about the attack just prior to a fundraiser near Philadelphia, and without more details about the missile strikes, Biden said he’d only speak briefly about the event, according to the pool report.
"What’s happening in Iraq and Iran today was predictable," Biden said. "Not exactly what’s happening but the chaos that’s ensuing ... And I just pray to God as [Trump] goes through what’s happening, as we speak, that he’s listening to his military commanders for the first time because so far that has not been the case."
Earlier on Tuesday, Biden delivered a forceful rebuke of Trump's foreign policy agenda -- calling the president "dangerously incompetent" and arguing his decades of experience position him to be best suited to assume the presidency on day one.
"President Trump has no strategy here, it seems to me. He has no endgame. And here’s the hardest truth of all: His constant mistakes and poor decision making have left the United States with limited options of how to move forward," he said during a foreign policy address in New York City, prior to the missile attacks in Iraq. "There is a smart way to counter them -- to counter Iran -- and a self-defeating way. Trump’s approach is demonstrably the latter."
Hours after the attack was first reported, Minnesota Sen. Amy Klobuchar said she was "closely monitoring what is happening in Iraq," before adding, "we must do all we can to protect our servicemembers and Americans at risk."
"I think we all know all is not well because we have an escalating situation with Iran and I hope that we're going to be able to step back now and use this precious moment to work with our allies" to "de-escalate" the situation, Klobuchar said on MSNBC late Tuesday, adding to her initial comments.
A Pentagon spokesman estimated that there were more than a dozen missiles launched from inside Iran targeting U.S. military sites in Iraq on Wednesday morning, local time. The facilities include Erbil in northern Iraq and Al Asad Air Base in western Iraq, a U.S. official said.
The rapidly unfolding development comes only days after a U.S. airstrike killed Iran's top Iranian military commander in the Iraqi capital of Baghdad. The move marked a major escalation in months of tension between the U.S. and Iran, despite the White House's efforts to say the strike headed off an "imminent" attack. Trump told reporters Tuesday that the U.S. "stopped" Soleimani as "he was planning a very big attack and a very bad attack, for us, and other people," but did not elaborate on the nature of that attack or on the evidence the U.S. had that attacks were "imminent."
As early reports of the ballistic missile attack unfolded, Hawaii Rep. Tulsi Gabbard, who often rails against "regime change wars" on the stump and has made foreign policy a central tenet of her campaign, told supporters at a town hall in New Hampshire, "Six rockets have been fired in to Al-Asad Air Base, and it's unclear at this point, what the casualties are, if any, and we're going to get updates here as we learn more information."
"I want to start with that, first of all, in just asking everybody just to -- to pause for a moment and send our good wishes and prayers to our men and women in uniform who are in harm's way today, as we are gathered here. These are men and women from all across our country who made a choice to serve."
At a fundraiser in Dallas late Tuesday, Pete Buttigieg, a former U.S. Navy intelligence office and mayor of South Bend, Indiana, underscored the stakes of the 2020 election, and the geopolitical implications of another four years of Trump, following the attack.
"It’s one more reminder of what is at stake in this election," he said. "As if we needed one more reminder that this is not a show and this is not a game. This is not about a horse race or who’s looking good or who got the best zinger off in the debate, this is a life or death process about the future of this republic."
"It will be weeks or months before we know the shape, the full shape of Iranian retaliation," he continued.
Several other Democratic competitors also sent prayers to Americans under siege in Iraq, including New Jersey Sen. Cory Booker, a member of the Senate Foreign Relations committee and one of the first candidates to react, who said, "Praying for the safety of our troops and personnel in Iraq right now."
Former Massachusetts Gov. Deval Patrick echoed that sentiment, tweeting, "Americans and our allies are under fire tonight. Let us keep them and their families in our prayers."
Colorado Sen. Michael Bennet wrote, "American servicemembers are under attack. I'm monitoring the situation and thinking of the brave men and women serving in our military tonight."
Entrepreneur Andrew Yang also invoked a similar tone saying, "Our thoughts and prayers are with our brave men and women serving in Iraq and keeping our country safe - may they be safe and secure and see their families again."
Author Marianne Williamson, too, said she was praying "for peace for all the world."
"Iranian rocket attacks on U.S.bases in Iraq. Prayers for safety of all military personnel. Prayers for wisdom for all governmental personnel. Prayers for peace for all the world," she said.
Vermont Sen. Bernie Sanders, who has not yet commented on the Iran attack, most recently told CNN on Monday, "Once you start this business of a major country saying, 'Hey we have the right to assassinate,' then you're unleashing international anarchy."
Sanders has often invoked the escalating tensions with Iran to share his long-standing aversion to war and continue his critique of Trump's handling of international affairs.
"As I said yesterday, I am deeply concerned that President Trump's actions represent a dangerous escalation that brings us closer to yet another disastrous war in the Middle East, which is exactly what we do not need," he told a room full of supporters in Iowa over the weekend.
ABC News' Ben Gittleson contributed to this report.