Battle-Interrupting Shout

Here are the two opposing armies, staring each other down, grinding their teeth, preparing for a massive and hopefully glorious battle. At some signal or instigation, they convene, with the glint and clash of steel and the rising of dust. But suddenly, rising over the noise of battle, and usually right before any actual fatalities occur, a voice is heard:

"STOOOOOP!"

It's The Captain, or the Old Master, or someone else of high authority. Alternately, it's an innocent bystander, probably a child, or someone else who is involved with whatever reason the two armies have for wanting to fight in the first place. Whoever they are, they've probably placed themselves physically between the two sides, with arms outstretched and palms facing out towards either force. And, at least most of the time, the cry for cessation is heeded.

Which is a little bit hard to swallow. Leaving aside the question of how one person's voice could be heard, simultaneously, by everyone, over the noise, many of them (especially the bad guys) would have no reason to obey the command and probably use the hesitance of the other side to seek an advantage.

As it is, both sides tend to pause at least temporarily to see what this interloper has to say. Often, the designated baddies will ignore it, or their commander will Kick the Dog by commanding the attack to continue, or by actually attacking the speaker.

Anime and Manga

Done once in Rurouni Kenshin in the Raijuta mini-arc.
This happens a lot in the Macross series, usually by way of a pop song, as music is utterly confusing to most aliens and causes disruption, allowing the humans to attack with little repercussion. It has also been used to resolve conflicts more peacefully, however.
Done twice by Nia in Tengen Toppa Gurren Lagann when Adiane attacks. It works the first time because she's a princess, and seems to work the second time until Adiane informs Nia that her father has abandoned her, and takes her hostage.
On One Piece, this happens not one, but many times. First, it is subverted by Princess Vivi as per Rule of Drama when she tried to stop the revolutionary army from attacking the royal palace. Played remarkably straight by Luffy later on: when he screams for a crowd to stop he can actually cause an entire battlefield to faint and froth at the mouth. Turns out it's a side-effect of his superpower.
- This is also Coby's undisputed Crowning Moment of Awesome during the Marineford war - Whitebeard and Ace have been killed, and now the marines and pirates are simply wearing each other down and taking enormous losses on both sides. Coby stands up in the middle of the battle and screams, bringing the battle to a temporary halt and revealing that he possesses shades of the same superpower as Luffy. Bonus points for facing down Akainu without blinking immediately afterwards.

Film

Happens frequently in the Pirates of the Caribbean films, often with Jack as the peacemaker. For instance, in On Stranger Tides, he halts a row between pirates and Spanish soldiers to point out that none of them have any real quarrel with one another even if their leaders do. This sways exactly one pirate, while everyone else goes back to fighting.
At the end of Ace Ventura: When Nature Calls, Ace prevents a war between two neighboring African tribes over the theft of a sacred bat, by running between the armies with it in his hands, shouting its name (which according to the tribes' traditions, requires all who hear it to kneel).
The Man Who Would Be King priests walk across the battle field and the War stops.
Averted in the movie Rustlers' Rhapsody, when Rex shouts until the gunfight stops, only to realize that it stopped because everyone had already shot each other.
Done twice by the hero in the Western parody Support Your Local Sheriff, once during a restaurant brawl, and then in the climatic gunfight.
Done by the giant Easter Island head in Night at the Museum when the museum was in anarchy.

Literature

In The Hobbit, Gandalf appears between the Dwarven, Elvish, and Human armies as they move to battle each other:

Thorin: Move, you old fool.
Gandalf: 'Old fool?' [revealing himself] HALT! [The armies hesitate] I would speak with the Kings.

In The Prophecy of the Stones, Opal and Adrien end a battle this way.
Jingo has Vimes and Carrot do this (and it works, since Carrot's so Lawful Good he practically warps reality to make everyone else the same).
- Another Discworld example, which lampshades how unlikely this is, in Monstrous Regiment where it is noted that when someone shouts stop it takes a bit of time to happen on account of everyone being a bit busy with all the fighting to listen.
- However, in the same book, Jackrum did supposedly stop a bar fight once by belching.
In Stephen King's The Stand, a nameless sergeant attempts to do this and immediately gets riddled with bullets from both sides.
In the beginning of The Malloreon, Belgarion is called upon to stop a war involving some of his friends. Both armies being Arendish, he decides he needs more than just his voice when stepping inbetween the opposing forces, so he calls down a thunderstorm. They stop fighting.
In Harry Potter and the Order of the Phoenix, Harry attempts to stop an argument between Snape and Sirius which is on the verge of turning into a duel, by shouting placing himself between them. This has little effect, but luckily the two combatants are distracted a moment later.
This is how Shadow prevents a war between the old and new gods in American Gods, although he doesn't shout. He just casually explains how they've all been had and they go home.

Live-Action TV

On one episode of The Cosby Show, Rudy tells Cliff a story she wrote about two countries going to war. At the end, just when the hostilities are about to start, a little girl shouts "Stop", and they stop. The end.
From Classic Doctor Who:

The Doctor: I'll deal with that later. First, I've got to put an end to this bloodshed.
The Brigadier: Oh, how, Doctor? Stand in the middle and shout stop?
The Doctor: Good idea.
One scene later and the Doctor is in the middle of the battle shouting "STOP!", and everyone does.

Myth and Legend

The Ur-example is the Sabine women who, when their dads FINALLY get around to trying to rescue them, run between the two armies holding up their babies (apparently it took the Sabines at least ten months to get their act together) and demand their menfolk make nice.

Opera

Otello: "Abbasso le spade!"

Video Games

In Magicka, Vlad intervenes in boss battles multiple times, and in one instance he stops you from killing a not-actually-evil boss by yelling "STOOOOP!"
Commander Shepard can halt a Geth-Quarian war in Mass Effect 3 with a carefully-timed rant about how the entire war is the Quarians' fault, prompting an implied Heel Realization by Admiral Han'Gerrel and an order to stand down.
Rayquaza from Pokemon Emerald does this with a roar, causing Kyogre and Groudon to stop fighting.

Western Animation

In the Smurfs' Animated Adaptation of the comic book story "King Smurf," all that is needed to stop the Smurf civil war is for Papa Smurf to return and roar, "Stop!"
In the second part of the My Little Pony Friendship Is Magic pilot, the ponies are attacked by a manticore in the Everfree Forest. Fluttershy tries to get the other ponies to stop fighting it, but they don't listen until she yells "WAAAAIT!"

Real Life

Julius Caesar's Gallic Wars assert that Celtic Druids could do this trick.