AMES, Iowa – The Islamic State is a “uniquely effective” adversary that will be defeated with a combination of American leadership and global participation, Democratic presidential front-runner Hillary Rodham Clinton said Sunday.

Following up on remarks she made at the second Democratic debate, Clinton told a crowd of Iowa Democrats that the deadly attacks in Paris, along with other recent terrorist attacks in civilians, was “absolutely unacceptable and cannot stand.”

The former secretary of state and one of her Democratic challengers, former Maryland Gov. Martin O’Malley, addressed a crowd of central Iowa democrats over a barbecue lunch. Clinton’s principal challenger, Vermont Sen. Bernie Sanders, sent author and liberal firebrand Cornel West in his place.

“The attacks in Paris, as I said last night at the debate, are a sobering reminder of the challenges and the threats that we face and the importance of American leadership,” Clinton said.

“This is a worldwide fight,” she continued. “I know America has to lead it, but we cannot and should not do it alone.”

During the debate in Des Moines, Clinton had called out Turkey and Persian Gulf countries for being wishy-washy in commitment to confronting the Islamic State. The al-Qaeda offshoot, also known by the acronyms ISIS and ISIL, has grown to be the most potent international terrorist threat.

French President François Hollande has blamed the group for the Paris attacks. If the Islamic State is behind the attacks, it would mark the boldest assault yet on Western targets outside the Middle East.

On Sunday, Clinton said allies and partners have to called “off the sidelines” to help.

“I know how difficult this will be, but I also know that we must be resolute and use both our strength and our smarts in combating this unusually effective threat,” Clinton said.

“It’s the first Internet-fueled terrorist group. ISIS is very agile and quite adept at using the Internet to propagandize, to recruit, even train,” Clinton said.

American and other counter-terrorism officials have pointed to the group’s effective and often innovative use of online recruiting and propaganda as one of its greatest strengths. Grisly videos showing decapitations of American and other hostages are credited with making the group widely known and attractive to disaffected Muslims far beyond the chunks of Syrian and Iraqi territory group now holds.

“Attacking Paris, the city of light, reminds us that there is no middle ground in going after these terrorists,” Clinton said. “I will work every day as your president to build a stronger, safer, fairer and more prosperous America because if we are not strong at home in taking care of our own people, we cannot do what we need to be doing to lead the world.”

Fomer president Bill Clinton, who had accompanied her to Iowa for the debate, made a surprise appearance at the Sunday rally.

O’Malley stuck largely to his stump speech about principled and innovative leadership, using examples from his time as Baltimore’s mayor and as governor. He joked that when a candidate polling at 5 percent claims that things are going well, his audience may well think “there’s a fine line between delusion and imagination,” but pledged to keep pressing his case.

West exhorted the crowd to rally to Sanders’s cause of racial, economic and social justice, and made several arch references to Clinton’s late adoption of liberal positions Sanders held before her.

“I’m so glad to hear my dear sister Hillary Clinton sounding so progressive, and I salute her efforts,” West said to laughter. “But it’s lip service if you don’t come to terms with your actions,”

Sanders remained in Iowa on Sunday but skipped the event in Ames. He had a pair of campaign events scheduled later in the day — a round table discussion with family caregivers in Des Moines and a town meeting in Indianola.