At one point, they were rumored to be friends.
When Briana DeJesus joined the cast of Teen Mom 2 for the show's most recent season, Kailyn Lowry seemed to be the only veteran cast member who didn't have a problem with it.
That all changed when Briana began dating Javi Marroquin, Kailyn's ex-husband and the father of her second child.
These days, Bri and Kail are bitter enemies, and it seems there's no end to their feud in sight.
Check out our rundown of the latest shots fired in this increasingly brutal war or words:
Briana & Javi: Getting Serious?
What started as a fling appears to be developing into a serious relationship. Briana and Javi have been exchanging flirty messages on Twitter, and Kailyn is understandably less than pleased.
Kailyn Lowry: Not Having It
Kail has moved on from her marriage to Javi, and she's doing her own thing these days. But it seems she's not a fan of the way Bri and Javi have been flaunting their happiness (or feigned happiness) publicly.
Is It All a Game?
Some fans suspect Bri and Javi aren't serious about their relationship and are only posting about it so frequently in order to get under Kail's skin.
Bri to Javi: Let's Get Married!
For some fans, the final straw was this tweet, in which Bri made reference to an earlier conversation that she and Javi had had about getting married.
Briana: Just Kidding About the Marriage Thing
In response to disbelief from fans, Bri later clarified that she was just joking about the marriage comments.
But the Feud Is Real
Shortly thereafter, Briana kept the feud fires burning by calling Kailyn out with some screenshots of a text conversation between Lowry and Marroquin.