Friendly War

Two or more characters or groups are involved in a "conflict" that they at first glance seem to take very seriously. Yeah, there's a huge war going on here. They really hate each other. Well, except that they don't. It's just a game, and there's actually no hard feelings involved. They simply fight each other because they know they all enjoy it and are comfortable to trust that no one gets hurt for real. They might even have a Safe Word.

If everyone involved makes sure no one gets hurt, it can be narrowed down to a Harmless War. However, not all friendly fighters are so considerate. If they are in it For the Evulz, the collateral damage might be considerable. (For the other kind of "Harmless War", see Non-Lethal Warfare and Nobody Can Die.)

Can lead to Abuse Mistake, as people don't understand that the whole thing is actually friendly. If any of the people involved are lovers, then this "conflict" is very likely to overlap with Casual Kink. Outsiders might view it as The Masochism Tango or Belligerent Sexual Tension, and in the latter case it might be entirely intentional: the lovers keep their relationship secret behind a role-played façade of constant bickering.

Compare & Contrast Running Both Sides, where the faux enemies aren't merely friends — instead, they are actually the same person.

Anime and Manga

 * Dog Days The war between the Republic of Biscotti and the Kingdom of Garettemore looks more like a game of war played by elementary students than anything else, people also get turned into furballs when they get hit.

Comic Books

 * Calvin and Hobbes are often at each other's throats, but it's usually only in good fun.
 * Calvin and Hobbes also gives us Calvin vs. Susie Derkins. After a shouting match, Calvin Breaks The Fourth Wall and says, "It's shameless how we flirt!" Could be Belligerent Sexual Tension if they weren't 6 years old.
 * This is the payoff of the DC Comics oneshot Superman and Batman: World's Funnest, a fight across realities between Bat-Mite and Mr. Mxyptlk.
 * In Doom2099, it is ultimately revealed that this was the cause of Doctor Doom's amnesia: His 'lover' had erased his memories as part of their "game". Subverted in that he was not happy about it afterward...

Film

 * In Razor Blade Smile, the heroine is a vampire who have spent the last 50 years fighting a conspiracy led by another vampire — who is assumed to be her ex-husband. In the end, after they have killed most of each other's minions, it is revealed that they are still lovers and that the entire conflict was just for fun.
 * In Attenberg, it is likely that much of the conflict and cruelty between Marina and Bella is actually neither conflict nor cruel.

Literature

 * In the novels about Bill Bergson ("Kalle Blomkvist" in the original language) we have "Röda Och Vita Rosen": The main characters have divided themselves into two teams that constantly oppose each other just for the hell of it. Often they fight over some destigated treasure, which they only want because they don't want the other side to have it. Of course, they don't let this struggle get in the way of the actual plot.
 * In Peter Pan, the Lost Boys and the Indians take turns attacking each other as a game. It turns serious in the Disney version when the Chief accuses the Boys of kidnapping Tiger Lily, who was actually taken by Captain Hook for the purpose of trying to get the location of Peter Pan's hiding place out of her.
 * In the Anne of Green Gables series, Anne and Gilbert's academic rivalry is played up as this, as least on Gilbert's side. To Anne, though, it is taken very seriously.

Live Action TV

 * The prank wars between the Winchester brothers of Supernatural are not so harmless at times, but certainly friendly.
 * Out of universe example: The regular 'wars' between factions of the Forever Knight fandom. They're essentially round robin style fics that include the fans and the characters and factions kidnapping items and characters and things like that.

Mythology

 * This is one of the appeals of Valhalla in Norse Mythology: Party all night, fight all day. Casualties don't matter, they're only temporary. Well, until Ragnarok, anyway, which they're training for.

Tabletop RPG

 * Dungeons and Dragons adventure EX2 The Land Beyond The Magic Mirror: The PCs can encounter a huge lion and a gigantic unicorn fighting each other. They're actually having a contest to see who will receive a valuable crown.

Video Games

 * In Runescape, the neighboring island nations of Miscellania and Etceteria are in a state of perpetual war, despite neither side having an army and nobody ever actually fighting.

Web Comics

 * In Order of the Stick, we have.

Web Original
"Grif: Let me get this straight. We're in the middle of an intergalactic struggle for control of the universe... and you guys are taking a few days off to go on vacation? Church: We'll be back in a few days, guys. We can start the war back up then. Grif: Okay; have fun. Church: And while we're gone, don't drive our tank. Grif: Fine! Church: And no parties!"
 * Red vs. Blue, though it later turns out that

Western Animation
"Thor: You see Billy, there's no need to worry. We fight for fun in Valhalla. And if we are slain, we end up back here in the feast hall. Billy: Cool! Let's eat!"
 * The Grim Adventures of Billy and Mandy: Much like the Valhalla example above, Billy finds himself in the Asgard world after being transported via an electrical discharge on Grim's scythe (switching places with a norseman, no less). He enjoys the feasting, but tries to convince Thor that they didn't need to fight. They both die on the battlefield, but only for a few moments.