After claiming responsibility for the Paris terrorist attacks last week, ISIS has a new foe.

Hacker collective Anonymous posted a video Saturday on YouTube in which it declared a cyber war on ISIS. In the nearly two-and-a-half-minute video, a person wearing the group’s signature Guy Fawkes mask read a statement in French promising that the hacktivist organization would attack ISIS in cyberspace with the ultimate goal of weakening the terrorist organization.

“Expect massive cyber attacks,” the person said. “War is declared. Get prepared. Anonymous from all over the world will hunt you down. You should know that we will find you and we will not let you go.”

ISIS has claimed responsibility for the horrific attacks that killed nearly 140 people and left hundreds more injured on Friday. The attacks prompted the French government to go on the offensive against the group. Prime Minister Manuel Valls confirmed on Monday that French authorities had conducted more than 150 raids and completed a bombing campaign against suspected ISIS encampments in Syria.

Anonymous, however, has its own plans. And as history has shown that it is not one to be taken lightly.

Anonymous, made up of an unknown number of loosely connected individuals, is believed to have been founded more than a decade ago on the online forum 4chan. The organization was firmly principled when it was founded and remains so, says Ben FitzGerald, cybersecurity expert and technology director for the national security program at the Center for a New American Security.

“Anonymous has its own set of values and it’s going to defend those things aggressively regardless of whether it disagrees with the CIA or ISIS,” FitzGerald says. “It doesn’t align itself around familiar paths.”

Anonymous gained notoriety in 2008 following its cyberattack on the Church of Scientology. Soon after, the group went on to attack any organization or issue that it believed had wronged others. Indeed, the organization’s attacks have targeted everything from governments to illegal pornography sites. Anonymous has also been credited with attacking the Vatican, the CIA, and San Francisco’s BART transport system.

“Whether you agree or not, they have a clear set of principles,” FitzGerald says. “They’re not nihilists; they believe in things and take action on those beliefs.”

By 2012, Anonymous had become a household name in the technology world. It also built up a track record for impact. In 2012, Time, which is published by the same company that owns Fortune, named Anonymous to its list of the 100 Most Influential People of 2012, saying that its attacks on networks had “earned its place on the list.”

While several similar hacktivist organizations have cropped up in recent years, few have been able to achieve the same level of notoriety as Anonymous. It is also among the more divisive of hacking groups, with some saying that it’s working in the best interests of the greatest number of people, and others arguing that it’s an illegal organization that must be stopped. Indeed, many law-enforcement officials around the world have targeted Anonymous and arrested dozens of alleged members.

Still, Anonymous has pressed on with no signs of slowing down. Just last year, after the shooting of Michael Brown in Ferguson, MO, Anonymous reportedly launched attacks on the city that took down e-mail servers and the phone system. The group last year also declared cyber war on the Ku Klux Klan.

“Anonymous is absolutely not slowing down,” says FitzGerald. “As they often say, they are ‘legion and they are hydra.’ They are based in multiple countries. They don’t have a directory of who everyone is. It would be very hard to track all of those people down.”

Anonymous, in other words, is fully capable of launching successful attacks—and ISIS knows it.

After the attack on Charlie Hebdo in January, Anonymous posted a video that said it would attack terrorists in connection with the killings. Soon after, it brought down sites allegedly connected with terrorism (including a dating site for ISIS members) and claims to have taken down tens of thousands of Twitter accounts of people suspected of having ISIS connections.

This, though, is different, Anonymous says. It warned ISIS after the latest attacks to “get ready,” adding that it doesn’t “forgive and we don’t forget.” While Anonymous did not reveal any details about planned attacks, the threat could be serious. Given its history.

“We should expect Anonymous to target ISIS members online and make ISIS member information publicly available,” FitzGerald says. “Anonymous will go after online personas and ISIS websites.”

Ultimately, however, it’s unknown how big of an impact the attacks will have on ISIS. FitzGerald argues that while ISIS is an Internet-savvy organization, Anonymous’ efforts may prove more of “a nuisance rather than a threat.” Regardless, he says ISIS needs to watch out.

“I think Anonymous will absolutely make good on those threats,” he says.

Sign up for Data Sheet, Fortune’s daily newsletter about the business of technology.

For more on hacking, check out the following Fortune video: