A type of Love Triangle — heavy on the triangle, light on the love. When a girlfriend leaves her boyfriend, and ends up in a relationship with the boyfriend's ideological rival, resulting in a vendetta between the two men.
It works with the genders switched around, too.
- This is one part of the relationship between Spike and Vicious from Cowboy Bebop. On the surface, they had one of these over Julia, resulting in the disintegration of their friendship and a lifetime of hatred toward each other. But this trope also applies to Vicious and Julia due to his Foe Yay for Spike, which is why he hates them both so passionately but (initially) spares their lives after they betrayed him.
- Turkey and Greece from Hetalia: Axis Powers were already on bad terms long beforehand, but them both liking Japan does not help one bit. According to France in the Christmas 2007 strips, it's bad enough that Japan spends most of his time with the two trying (vainly) to get them to be at least civil to each other.
- Brutal example in the School Days anime. Makoto starts out interested in Kotonoha, gets help from Sekai, and later falls for her, then goes back to Kotonoha at the end after he suspects everyone ditched him due to his lecherous ways by sleeping around with all the girls in the series, and Sekai's declaration that she's pregnant. He gets knifed to death by a jealous Sekai, who in turns gets killed by Kotonoha at the end. When you're in a Psychotic Love Triangle, it's not going to end well.
- In The Matrix Reloaded, Niobe used to be with the mystical Morpheus, now she is with the secular Locke. How much of the tension between the two men is a result of this and how much is due to their very different personalities and leadership styles is unclear since it all happened in the backstory. That Niobe keeps supporting Morpheus over Locke doesn't help matters much.
- The Prestige.
- Though the vendetta was pretty firmly in place before this came about.
- Played brilliantly in Spider-Man, where Mary Jane is constantly the center of the tug-of-war between Spider-Man and the Goblin Jr. AND their alter egos Peter Parker and Harry Osborn.
- Alexandre Dumas' The Count of Monte Cristo. This is particularly pronounced in the latest Film of the Book, in which Mercedes is one of the key reasons behind Fernand's plot to put Edmond in prison.
- Harry Potter: Harry's mother used to be close friends with Severus Snape when they were children; he had an obvious crush on her (although no one but Dumbledore seemed to know), and a deep and abiding hatred for his then-rival, James Potter. Since Harry reminds Snape of his father, you've got some serious hostility there.
- Also completely played straight since Lily initially didn't like James for being arrogant. However, her friendship with Severus was gradually breaking apart before it finally broke when Severus whipped out the Fantastic Racism card on her after she tried to interfere with him and James. She ended up dating the now less hot-headed James, whom she eventually married and had Harry with.
- There is, of course, a Buffy the Vampire Slayer example. Tension over Drusilla's affections is the main source of the feud between Angel and Spike for much of the second season, even leading to Spike helping Buffy at the end of the season in the hope he'll get Drusilla back.
- Ironically, Spike himself ends up romantically entangled with Angel's love interest, Buffy, in later seasons. These guys have VERY similar tastes in women.
- Greek. Cappie's carelessness and jealousy leads him to beat up his best friend and roommate Evan for taking Casey to a formal when he failed to show. This leads to the pairing of Casey and Evan at the beginning of the series and the bitter rivalry between Evan and Cappie's respective fraternities ever since.
- Smallville: Lex and Lana are an item for a time, resulting in upped tensions between Clark and Lex.
- In the Phoenix Wright: Ace Attorney series, one of the cases involves a couple of dueling celebrities (one of which is a Jerkass and the other plain evil) and when the current fiancé (the Jerkass) found out that he had essentially gotten his rival's leftovers, he immediately called off their engagement. The rival had told him this just because he knew it would make his rival angry and this only escalated the rivalry between the two of them. The woman in the meantime? Tired of being The Chew Toy, she kills herself.
- Onmyōji: Shuten-dōji is mad at Seimei for apparently having seduced his unrequited Love Interest Momiji, much to the dismay of the totally innocent Seimei who a/ doesn't remember a single shit he has done in the past, and b/ truly did briefly met Momiji before, but did not seduce her. The one having done that was someone looking similar to him, who is also a Literal Split Personality of himself.
- Missi and Emily in Misfile to the point where Ash tells both of them to bug off until they can be decent to each other.
- Dr. Horrible's Sing-Along Blog: the Love Interest is Wide-Eyed Idealist Penny, the opposites in this case are the Villain Protagonist Dr. Horrible and the Hero Antagonist Captain Hammer; Captain Hammer is far more into how much it torments Horrible than he is actually interested in Penny. When the Captain rubs it in explicitly and gloats about taking her just because Horrible wants her, the revelation pushes him past I Want My Beloved to Be Happy and convinces Dr Horrible to Murder the Hypotenuse. He argues to himself (in song) that Murder Is the Best Solution both to claim his love from a man who is only using her and to simultaneously prove he's a real villain worthy of respect and fear. She dies instead, leading to Dr. Horrible being inducted into the Evil League of Evil, his major goal throughout the story.
- LaBarbara Conrad from Futurama has been married to both Hermes Conrad and his limboing opponent, Barbados Slim. In this case, they are opposites mostly in terms of Conrad being a short, chubby bureaucrat and Slim being an "ebony adonis" professional athlete (though Hermes is usually the more talented one in their common sport of limbo).
- Happens in Total Drama World Tour with Duncan, Courtney and Gwen.