They’ve attacked Kelly Osbourne, ganged up on Piers Morgan, and made off-color jokes about a member of the Black Keys. No celebrity is off-limits: They’ve gone after Twilight princess Kristen Stewart, crooner Chris Brown, and singer Christina Aguilera. When it comes to rivalries, there are few fiercer than that of the fans of boy bands. Say something bad about One Direction, and the Directioners will cut you.

The celebrity fan base, once something to admire, is now something to be feared. These so-called fan armies rally around their favorite artists, and step up to the plate to defend them in vicious online feuds.

From Justin Bieber’s Beliebers and Nicki Minaj’s Barbz and Kenz to Haley Reinhart’s Haliens, fan armies have the ability to wreak havoc on Twitter with a single hashtag.

When news leaked that Beyoncé lip-synced “The Star Spangled Banner,” her “Beyhive” came alive in (deluded) defense of the star. Wrote one member of the Beyhive, @_OH_SO_PRETTY: “To all those Beyonce fans who think she was lip-syncing at the Inauguration…you’re not a true member of the #BeyHive.”

Not even stuffy British newscaster Piers Morgan was above the fray of fan armies, as he found out when he made a derogatory comment about soccer star David Beckham. Apparently, teeny-bop boy band One Direction’s fans, dubbed “Directioners,” had Beckham’s back. Faster than you could hit “send,” the phrase “Piers Morgan is smelly,” was trending online.

As Morgan told Mario Lopez, “Suddenly, I have 35 million girls tweeting me and they’re all doing the same thing, ‘Piers Morgan is smelly.’”

If only the warring political parties could harness such power.

In the era of Twitter and Tumblr, fans are an ever more powerful and cruel force than they were when fan clubs were quaintly run via snail mail out of a devoted fan’s home.

“In the early to mid ’90s, we actually wrote letters to each. Like, handwritten letters,” said Rachel Chang, the editor in chief of teen magazine J-14.

Back then, fans weren’t a mobilizing force like they are now—in part because they weren’t instantly connected. Fans were “probably in their bedrooms by themselves and had very few people to talk to,” she said. “Nowadays, it’s so different because it’s all via Twitter.”

Brenna Ehrlich, senior writer and editor for the O Music Awards, agreed. “Definitely, with the advent of social networks, it’s become a lot easier for these fans to find each other. You don’t really need to be a computer nerd or someone who knows how to code to be able to connect.”

Indeed, fan armies are so hyperconnected—and mobilizing—they now have their own categories at award shows, including O Music’s Awards, which awarded Tokio Hotel’s Aliens its “Fan Army for the Win” prize. The announcement on Twitter garnered more than 600 retweets in a few hours. Meanwhile, MTV’s Year in Review recognizes “Best Fans.” (Winner: Demi Lovato’s Lovatics.)

“I think the best fan armies are the ones who really have some kind of genuine engagement with their fans and don’t just tweet at them like, ‘I love my fans’ and don’t just take a picture of themselves on Instagram pouting into the camera,” said Ehrlich.

But fan armies have mostly gained notoriety for their propensity to harass and cajole. Perhaps most ironically, the fans of Lady Gaga, called Little Monsters, have been particularly legendary for the full court press they put on people they don’t like.

When Patrick Carney of the Black Keys cracked wise about Lady Gaga in 2011, Gaga’s 33 million–strong army set into action, besieging him with hate mail. Wrote one angry fan, “Who the f--- is @patrickcarney? The kid in the wheelchair from Glee??”

And a few weeks ago when Gaga posted an open letter on her website criticizing Kelly Osbourne for her part on the fashion criticism show Fashion Police, in which she pokes fun of celebrity’s outfits on the red carpet, her critique was marred by the fact that she had to apologize for the behavior of some of her Little Monsters who had attacked Osbourne when she criticized the singer for not showing up on the red carpet at the 2012 Grammy’s.

After the litany of attacks, Osbourne tweeted, “@holyh00ker i have been told to die,suck dick, get raped & that i look like i have aids by @ladygaga fans no wonder they get called bullies.

Billboard’s editorial director, Bill Werde, weighed in on the feud and fan army belligerence on Twitter. “The nasty side of fan armies is that it’s a form of bullying. Every one of us owns our chance to make the world better, one act at a time.” He added, “Shame that the actions of some of Gaga’s twitter fans open her to charges of hypocrisy re her Kelly Osborne letter.”

In some cases, fan armies can even turn against their artist and vice versa, as Nicky Minaj’s Barbz did when she deleted her Twitter after one of her fan sites leaked her album. She wrote, “And that’s exactly why I’m paying the barbz DUST right now! And deleting my twitter,” she wrote. “Smdh—don’t cry 4 me Argentina. “Like seriously, it’s but so much a person can take. Good f*****g bye.””

But those who are most familiar with fan armies say that most of the time it’s all in good fun.

“I just think it’s a fun way for the stars to interact with their fans and it seems like a team,” said Chang. “It’s almost like sports teams against each other.”

And, like most sports teams, the rivalry is serious, but not hurtful.

“Unless they’re ganging up on some teenager in the Midwest or something, it’s really not a problem,” said Erhlich. “Because if someone like Patrick Carney is going to say something about Lady Gaga in the news, then I don’t think he’s going to really mind that a bunch of fans are going to talk back at him. Because it’s in the public sphere. So I think when it’s that kind of bullying, like fans bullying other famous people, it’s not really that much of an issue. I don’t think I’ve heard many instances of fans bullying people who would be significantly hurt by that.”

Kayleigh Roberts, the online editor of Bop and Tiger Beat, had her own take on the phenomenon. “Fan armies can devolve into bullying,” she said. “This is most prominent in cases like the Directioners vs. Rushers or Directioners vs. Swifties, when one group feels like another is somehow threatening the celebrity they love. When fans get out of hand, however, most celebrities are really responsible about diffusing the tension with a tweet encouraging the fans to play nice. Justin Bieber has had to do this more than once and he’s great about setting a good example for his fans and reminding them to stay positive.”

Indeed, in her letter to Osbourne, Gaga had to acknowledge that her fans had gone overboard. “While some of my fans have learning to do, most of them share the same values as I, and it’s what bonds us together. And that bond is strong.”

In the case of fan armies, maybe a little too strong.