Tengen Toppa Gurren Lagann, At the Battle of Teppelin in episode 14. Dai-Gurren is about shot down by Cytomander, when a barrage slams into them from another direction. Cut to a huge army of lesser battleships and mecha, all hijacked by people from various villages who saw and heard of Team Dai-Gurren’s battle. This leads to an awesome shot of the entire army arrayed and heading into battle against Teppelin’s forces.
In One Piece, The Red-Haired Pirates for the Whitebeard Pirates in the Battle of Maineford. Noteable in that, instead of kicking ass, the threat of it, stands down the Marines, the Seven Warlords of the Sea, and the Blackbeard Pirates.
In Transformers Cybertron, after an Omega Lock-powered Galvatron has creamed Optimus Prime's team, the armies of Jungle Planet, Velocitron, Earth, and Gigantion all arrive to help. Lugnutz even persuaded the Ancient Decepticons to led a hand, on the grounds that they don't want the universe to be destroyed.
In Holyland, after Yuu defeats Katou, Shougo invokes this by calling the police so that Katou's thugs don't get to beat Yuu. Then in chapter 58, when Yuu is getting beat up by some nobodies, Iwado shows up and saves him.
When Celestial Being are getting swamped by A-Laws, and most of their weapons nuetralized by their anti-beam clouds. The Katharon forces and defectors from A-Laws soon come to their aid.
In season one, when Celestial Being is getting beaten back by an unprecedented team-up between the three rival superpowers. They carry out a relentless and well-planned assault that's overcoming the Nigh Invulnerable Gundams by sheer weight of numbers. Then the Trinity siblings show up, with three more Gundams of their own that nobody knew existed, beating back the enemies and allowing Celestial Being to escape unharmed.
One of the most memorable moments in Char's Counterattack (and some would say in all of Gundam comes at the end: Char is defeated, but half of Axis is still falling into Earth's atmosphere. With little choice, Amuro flies his Nu Gundam in front of Axis and tries altering its course by pushing the giant asteroid. Just when things look their bleakest, the Londo Bell forces join in (as do several of Char's men). Double Subverted: The focused mental energy of all those people working together combines with the black boxes in the Gundam and what's left of Char's machine to create a psychic field that pushes the "cavalry" aside...but it also pulls the piece of Axis back into space.
The Battle for Kabuki-cho in Gintama. The baddies make it clear that the Yorozuya need to pack it up and head their separate ways out of Kabuki-cho, going so far as to kill Otose (she got better). When Gin and company refuse and stand their ground against a huge gang of thugs, the other residents of Kabuki-cho (mostly people the Yorozuya had helped in previous stories) show up to fight at their side: the cabaret girls, the host club hosts, the fire department, the nutty robot engineer...and Hasegawa.
In "The Lord of Lorn and the False Steward", when news of what happened to the young lord arrived back at home, his father collected hundreds of men to arrive in force and rescue him — without, mind you, fighting.
Of course, this shows up more or less regularly in Lucky Luke.
Subverted, as most tropes in Lucky Luke eventually are, in an issue where the cavalry arrives too late to save a band of settlers from an Indian attack. The cavalry commander has a nervous breakdown when Luke informs him that the cavalry came too late. "But... but... the cavalry is ALWAYS on time..."
And the one time the adjutant reminds the captain that they have to leave now, and cut short the thank you scene, in order to be on time for the next rescue.
Done in one JLA story with the role of the cavalry played by the entire population of Earth, freshly given super-powers and ready to tear apart a primordial force of evil in order to help Superman.
Also done at an earlier point in that JLA run - with a tad more subtlety - when the entire population of Earth was mobilized by Superman to fight back against an alien White Martian invasion, using fire.
Plus, earlier in that story, Batman acts as a one-man cavalry in service to the entire, captive JLA. Given extra oomph since the aliens are convinced that he's no threat because he's "just a man".
Appears in a Star Wars Expanded Universe comic set during the Clone Wars where Anakin arrives just in time with a bunch of clones, and when someone rhetorically asks who it is, he says, "The Cavalry." I guess he's watched a lot of space westerns?
Namor's arrival at the end of the Civil War should count as one. Problem was, Iron Man had his own cavalry as well.
Downfall has one in Chapter 17- Later, in Chapter 20, Momo and a platoon from Squad Five arrive to reinforce Rangiku and the exhausted Squad Six detachment denying the Arrancar retreat to the Garganta.
In Legends the Avengers are surrounded by all the guns S.H.I.E.L.D. can muster. They affirm their resolve to stay where they are, Wolverine and Daredevil share a loving goodbye. Captain America gets the team ready to attack on his signal, and the leader of the S.H.I.E.L.D. troops gets ready to signal the open fire. Suddenly, their dead teammate falls down from the heavens, given the title of Thunderstrike for dying bravely. He heals up the sorcerer supreme who turns their enemy's bullets into sand. S.H.I.E.L.D.'s leader gives the order for them to prepare to fire again and then Thunderstrike summons the warriors of Valhalla, which for those of you not clued into Norse Mythology is the resting place for warriors who have died in glorious battle. Every. Single. One. Including the original Avengers. The S.H.I.E.L.D. agents promptly drop their weapons.
Pokemon Masters of Azeroth has a memorable scene at the battle for Icecrown Glacier. The dragons finally show up, en masse, as they had promised, just when it looks like the heroes are doomed.
In Tiberium Wars, the GDI defenders at the Pentagon are about to be overrun by the Nod forces being supported by Avatar warmechs, until the Mammoth Tanks arrive. Earlier in the story, the GDI Marines play this role when they rescue to troops trying to recapture the airbase.
During the Final Battle of Inner Demons, while the Element bearers and the Princesses are fighting through the hordes of Tartarus in order to confront Queen!Twilight, a large army of her undead Elite Mooks get past them and prepare to march on the defenseless Ponyville. The aged-up Cutie Mark Crusaders attempt to stop them, but don't seem to making enough of an impact... then the Royal Guard, commanded by Shining Armor and Cadence, show up and engage the undead in combat, keeping them from advancing.
Done literally in Cars 2when Sarge contacts his old military buddies to bring the entire British Army to rescue Mater and Lightning McQueen.
Played with in Ratatouille. Remy's clan of fellow rats shows up to help him prepare a gourmet meal in a restaurant.
Done rather clumsily, but nevertheless played straight in BIONICLE: The Legend Reborn — a gigantic army composed of Agori, Glatorian and battle chariots appears to help Mata Nui fight the Rock Tribe and the Bone Hunters... but you only see the main characters duking it out in the ensuing battle, and the army just disappears for the rest of the scene.
My Little Pony: The Movie: The Flutter Ponies coming in to defeat the Smooze. They pull this again in the TV series, showing up to defeat Erebus.
Film - Live Action
Every D.W. Griffith film made from about 1910 on. Sometimes The Cavalry is one man, sometimes it really is the Cavalry. And sometimes...well, see below.
Evil Dead: "The Red! The Red! Duke Henry and his men have come!"
Stagecoach, the movie that made a star of John Wayne, is a classic example, in which the cavalry appears exactly when all hope is lost, although it had been previously implied they would not be available.
The highly controversial silent film, The Birth of a Nation, which has every racing-to-the-rescue shot from every Western ever made. The only problem is that The Cavalry happens to be The Klan. (Griffith was obsessed with The Cavalry, and used whatever or whoever was conveniently to hand - causing occasional "WTF'ness".)
In Peter Jackson's The Return of the King, there is a cavalry overload at Minas Tirith, at their many variants. It begins with the city standing against the orcs. Then, the orcs are reinforced by the Big Damn Villain, the Witch-King, who confronted Gandalf. Then, The Cavalry (an actual real cavalry, the Rohirrim) comes to help the good guys. Then, a second cavalry, composed of Oliphaunts, comes to help the bad guys. And then, to secure the evil victory, comes yet another evil cavalry, the Corsairs... but it was a Cavalry Betrayal (to the evil side), as the Corsairs ships were actually used by the Big Damn Heroes Aragorn, Legolas and Gimli and the Army of the Dead.
The climax of the final battle in Saving Private Ryan is the arrival of the P-51 Mustangs that blow the German Tiger, which shrugged off bazooka shells, away before you even see them. Now that's a dramatic entrance.
Subverted in Zulu, when a sizable force of Boer cavalrymen turn up at Rorke's Drift ahead of the approaching Zulus, then gallop away to avoid being killed by those very same Zulus. The heroes don't take this well at all.
Word of God is it's a deliberate homage to the cavalry charge in a western with the Falcon coming out of the sun.
In A&E's "Horatio Hornblower" movie, The Wrong War, the heroes are trapped on a beach, facing the French army. The heroes form up in ranks, and fire off a volley at the French in desperation, and are completely surprised when their little muskets cause massive devastation. They turn around to see that their ship, "H.M.S. Indefatigable", has suddenly arrived and is shooting at the French. It's a great cavalry scene, if one sets aside the fact that ships' cannon of the period were so inaccurate that the ship had a better chance of hitting their own troops than they did the French. But their own troops were about to die anyway (unless they surrendered), so it was presumably worth the risk.
The Three Musketeers (1973). D'Artagnan has been separated from the other three Musketeers through attacks by the Cardinal's agents. As he's trying to return the diamonds to the Queen, he's accosted by several sets of Cardinal's guards. Finally, when he's outmatched by three of them, he's rescued twice: first by his servant Planchet in a polar bear costume, and then by the rest of the Musketeers whom Planchet lets in through a gate.
Parodied in the Bing Crosby / Bob Hope film Road to ... Rio: while the two heroes are busy tangling the villainess and her two henchmen, an ally (Jerry Colonna) and a bunch of reinforcements ride furiously toward the heroes' location. But then suddenly, the heroes have won and the villains are hauled off to jail without The Cavalry ever arriving. Colonna reins in his horse mid-gallop and cheerfully comments to the camera: "Well, we never made it. But exciting, wasn't it!"
In Shaun of the Dead, the titular character and his girlfriend (but not his best friend, who has been bitten and opts to hold off the zombie hordes) are saved by the cavalry in the form of the British Army who proceed to avert the standard zombie movie cliche and absolutely slaughter the zombies.
The Cavalry is generally a very happy concept. See the movie version of The Mist for an example where it really, really isn't. (In the original novella they never show up at all. Which, arguably, makes the ending of the novella happier. Or at least less fucking depressing.
The main characters of the Doom movie are, as confirmed in the dialogue, very explicitly the cavalry. Too bad it doesn't help.
Jurassic Park III. The Cavalry saves everyone's asses at the very end, including a character long thought eaten.
In the first Jurassic Park where the Cavalry appears as a Tyrannosaurus Rex.
Interesting evil version in 3:10 to Yuma, where the Affably Evil Ben Wade predicts throughout the film that he will be rescued from execution by his Quirky Miniboss Squad. There are several scenes showing them tracking down their boss, and sure enough, they ultimately come to rescue him, and Wade's status as a Magnificent Bastard is perfectly clear.
At the climax of Avatar, the wildlife of Pandora itself comes to aid the Na'vi, who are getting curb stomped by the human forces. Could also be Gondor Calls for Aid, as Jake asks for help from nature before hand, but is told it can only "protect the balance".
The actual "horse" riding Na'vi cavalry is rescued by the metaphorical cavalry.
Just as things look their worst in Sleepwalkers, Clovis the police cat leads an army of cats on a slow motion charge toward the villains' house.
Subverted in Time Bandits. While in the Fortress of Ultimate Darkness, Kevin and Ogg stay behind to distract Evil while the others go through the time doors to get help. Just when it looks like Evil is going to win, the others return with American cowboys, Arthurian knights, Greek archers, a tank and a laser-armed spaceship to save the day. It looks like Evil has had it, right? Nope. Evil uses his powers to annihilate each of the reinforcements in turn, and is only stopped by a literal Deus ex Machina, when the Supreme Being shows up and turns him into charcoal.
Subverted in Alien³. Weyland-Yutani has send a rescue expedition to Fury 161 which arrives by the end of the film, but they don't care at all about saving the prisoners or Ripley; they just want to retrieve the Alien.
Subverted twice in Jaws 2. After the shark attacks the teenagers and sends them adrift on a raft cobbled together of what's left of their sailboats, help arrives on two occasions, but they each fail in different ways.
A helicopter (the type that can land on water) arrives to tow the boats. The shark attacks the helicopter, eats the pilot, and the mayhem causes the rafts to collapse, leaving almost nothing left for the teenagers to stay afloat on.
Chief Brody later comes to the rescue. The shark attacks his boat as well, causing him to steer and crash it into the tiny island that was close by. As a result he has to find other means to save them.
When the main characters of The In Laws, one of whom may or may not be a rogue CIA agent, are in front of a Latin American firing squad during the film's climax, they're rescued at the last minute by the appearance of a small army of CIA agents. (They would have been there earlier, but "traffic was brutal.")
During the climax of Star Trek VI The Voyage Home, Enterprise is getting pounded severely by Chang's Bird of Prey, and is unable to fire back due to the Bird of Prey's cloaking device. Excelsior arrives shortly Enterprise's shields fail, giving Chang a second target to occupy his attention and buying time for Kirk's crew to finish modifying their seeker torpedo. Enterprise does still get badly mauled, but without Excelsior's timely arrival she would have been destroyed before Spock and McCoy could finish their work.
At the beginning of Star Trek: First Contact, Enterprise gets her turn as the cavalry, arriving in the middle of the battle with the Borg just in time to rescue Defiant and direct a decisive counter-attack to destroy the Cube.
Somewhat Zigzagged in Iron Man 3. The cavalry in this case is all of the Iron Man suits, basically a One-Man Army controlled by Jarvis except for the suits Tony uses. They're also torn through pretty quickly by the people who have Extremis.
More than one of the ugliest battles in Honor Harrington has occurred in the interval between the Cavalry arriving and actually coming in range to support her. Such is the danger of belonging to one of the few sci-fi writers who DOES have a sense of scale.
One of the earlier examples, Honor Of The Queen, had Honor needlessly pursuing a badly mis-matched battle with an enemy battlecruiser because she did not know the Cavalry had already arrived, due to her ship's sensor suite being smashed. The first indication she has that she is no longer out-gunned is when the enemy battlecruiser abruptly maneuvers to defend itself from an incoming Manticoran Missile Massacre.
The General: General Raj Whitehall only occasionally needs to be rescued by his cavalry, mounted on half-ton 'wardogs' ranging from collies to newfoundlands, bred to serve the purpose of horses in real life.
In the novel 1632, Grantville is being attacked by some ruthless mercenaries (who have been paid by Cardinal Richeliu to destroy Grantville since it threatens his plans). Although holding their own, they are slowly being outmatched, since their main army has been lured away and won't make it back in time. Then, in a Big Damn Heroes moment, the crazy Captain Gars and his men (who spotted the mercenaries and have been rushing to Grantville to help) add reinforcements to drive the mercenaries back.
In A Song of Ice and Fire, this happens twice. Once is on the Wall, when Jon Snow has been trapped by the wildling Mance Rayder on one side and the corrupt Janos Slynt on the other. It appears that he is in a situation with no way out, and the wildlings will overrun the Wall. Then The Cavalry quite literally arrives; Stannis Baratheon shows up with a thousand armored knights and puts the wildlings to flight. The other time is at the Battle of the Blackwater, when Mace Tyrell and Tywin Lannister show up in the nick of time to save King's Landing from Stannis.
Subverted with Ramsay Bolton, who arrives when the original rulers of Winterfell are besieging it to get it back from Theon Greyjoy. And neglects to mention that he's no longer on their side, destroying them easily by sneak attack. And when Theon, taking him to be The Cavalry opens the gates, reveals he's not on his side either.
The Riders of Rohan would appear to have this down to an art in The Lord of the Rings. They pull it off twice and, like any good Cavalry unit, know to time their arrival for the most dramatic moment. In the battle of Helm's Deep Gandalf and Erkenbrand lead a charge of Rohirrim infantry to aid the army riding out from the besieged citadel. In the siege of Minas Tirith the Rohirrim arrive just as the main gate to the city has fallen.
And the Rohirrim were given their lands because of their first most successful Cavalry moment when Eorl came to help Gondor all the way from the Northern lands.
The Riders of Rohan are amateurs when compared to the Eagles, however, who make a business out of showing up at the last moment to save the day, though some of the instances count more as an in-universe Deus ex Machina. They do it four times in The Lord of the Rings by saving Gandalf from Isenguard, saving him again when he's stranded in the mountains after killing the Balrog, at the battle for the Black Gate by attacking the Nazgûl, and then helping Gandalf rescue Frodo and Sam. They do it twice in The Hobbit, once to rescue Bilbo, Gandalf, and the dwarves from being treed by goblins, and again at the Battle of Five Armies. They make multiple appearances in The Silmarillion, swooping out of nowhere to save Maedhros who was chained to Thangorodrim, slashing Morgoth's face and saving Fingolfin's corpse, saving Beren and Lúthien from Angband, knocking Orcs off a cliff to save the survivors of Gondolin, and showing up with Eärendil to take down Morgoth's winged dragons in the final battle at Angband. Their leader Gwaihir seems to have made it his personal mission to pull Gandalf's fat out of the fire as often as possible, having done so at least five times. However, there's very little activity from the Eagles outside of this, since they are specifically at Manwë's command and Manwë doesn't wish to intervene too often.
Dale Brown's novel Silver Tower. The carrier U.S.S. Nimitz and its escort group have taken several hits from Soviet cruise missiles, and even more missiles are approaching. At the last second, the laser on theSilver Tower space station (which had been malfunctioning earlier) finally comes online and blasts the Soviet missiles out of the sky like clay pigeons, saving the ships from destruction.
In Fatal Terrain, the previously-secret Taiwanese Kai-Shan Military Complex is under Chinese bombardment when USN and USAF assets show up.
In Air Battle Force Hal Briggs is about to be on the receiving end of a Hind's weapons when Turabi and his Taliban men save his ass.
In Ben Counter's Warhammer 40,000Horus Heresy novel Galaxy in Flames, Tarvitz gets the loyalist Emperor's Children to disengage, and join him. Then the TurncoatLucius attacks him — but the Children arrive in time to stop him. AND then engage in the counter-attack that Tarvitz had planned.
In Jim Butcher's Dresden Files novel Storm Front, Morgan walks into the end of the climatic fight and swings his sword toward Harry, who is falling unconscious. He regains consciousness to find Morgan administering CPR; Harry had, after all, saved himself from falling into a maelstrom of magic by chaining himself to a burning building, and Morgan had to get him free somehow.
In White Night, Harry opens a Way in the climactic battle ... and massive reinforcements arrive. Nowhere near as many as the enemy, but such is the life of Harry Dresden. It turns out afterwards that he made a significant deal to get those reinforcements, though.
In Changes, Harry and his small group of allies are utterly surrounded by an army of the Red Court, and while they are holding their own, they can't hope to win against tens of thousands of vampires, half-vampire minions, and mercenaries. Then the Leanansidhe opens a lightning gate that allows the Grey Council (which includes both Blaffstaff McCoy and fucking Odin!) to teleport in along with an army of kenku.
Harry leads the reinforcements in Cold Days, in the form of the Wild Hunt.
Deconstructed in the Sienkiewicz Trilogy's With Fire and Sword. Yes, the cavalry comes, but two of the main characters have to go on a suicide mission through enemy lines to call for it, and one of them actually is killed. The other one also nearly dies of starvation and exhaustion.
In Chris Roberson's Blood Ravens novel Dawn of War II, they fear that the shadow the tyrranids have cast on the warp will prevent this, and they are fighting a Last Stand, but the Third Company arrives in time.
In Chris Roberson's Imperial Fists novel Sons of Dorn, the rest of the Imperial Fist forces return when the scouts have managed to draw out the full Chaos forces. There are even five survivors in the forces they left.
In Lee Lightner's Warhammer 40,000Space Wolf novel Sons of Fenris, the Dark Angels continue pulling out of the system even when a Chaos ship appeared. The Space Wolves go to fight alone — and the Dark Angels reappear, flanking the enemy. Whereupon they explain that they had intended that all along, but could not exactly tell the Space Wolves their plan over an open communication link.
Especially good because the Dark Angels and Space Wolves can't stand each other. Though it's more of a friendly rivalry.
Late in the fourth book of The Wheel of Time, Perrin arranges for Faile to get to safety while he prepares his village's Last Stand. Faile doesn't get herself to safety, but instead rallies the northern village to become this. The southern village did so of its own accord.
Rescuing Rand at Dumai's Well, the Asha'man appear to turn the tide.
Rand later rescues Bashere and co. at the Blight, drawing so much of the One Power that the muggles could sense it and he nearly started the Last Battle on his own.
Mat and Tuon pretend that there was a split in the forces of Good between the Aes Sedai and Seancha during the Last Battle. This allowed the latter force to sweep back in at a critical and dramatic moment.
The Sharans for the Trolloc hordes during the Last Battle.
In David Weber's In Fury Born a unit of the Imperial Cadre, the elite forces of the army, is betrayed by its intelligence officer and drops from orbit straight into strong anti-aircraft fire and prepared positions. They then have to fight across long distance and through many enemies in order to disable said anti-aircraft to allow The Cavalry (in this instance, Marines in ordinary dropships, as opposed to Cadremen in fast drop pods) to land. Out of 250-some original members of the unit, nine survive. Also a Crowning Moment Of Awesome.
In James Swallow's Black Tide, there are no less than three arrivals of the Cavalry: Vetch to save Rafen and Tarikus; Killian and the other prisoners to save Rafen, Vetch, and Tarikus; and Noxx and the squad to save the prisoners fighting the splices.
In Mikhail Akhmanov's Dark Skies, a human colony is under occupation by a race of Lizard Folk. The protagonist leads La Résistance in a daring attempt to behead the occupational forces by killing the Clan Elder using a cache of weapons hidden during the previous war. Unfortunately, the operation fails, and the rebels are being besieged by a massive force of enemy soldiers who do not value life (neither theirs nor ours). Just as the situation seems bleak, the rebels, who are holed up underground, hear explosions on the surface, which the war veterans immediately recognize as those used by The Federation fleet. While the protagonist attempted to call for help earlier, he didn't know when or if the help would come.
Subverted in the beginning of the novel when a similar attempt to be the Big Damn Heroes ends in disaster when the occupational forces turn out to be a lot more numerous than expected, and all but one rescuers are killed.
In the Legacy of the Aldenata, the ACS often serve in this role, particularly the units headed by Mike O'Neal, Jr. In one charge to the rescue in Gust Front, at the battle in Washington, DC he even plays Yellow Ribbon, the anthem for the US Cavalry, over the suit speakers.
In Harry Potter and the Deathly Hallows, the cavalry arrives during the Battle of Hogwarts: Harry seems to have died, Voldemort has set the Sorting Hat on fire while Neville is wearing it, the Death Eaters seem to have won... and suddenly there are four cavalries coming at the same time: an army of centaurs from the forest, Buckbeak the hippogriff leading a pack of Thestrals, the house-elves from the kitchens, and Professor Slughorn, an old man thought to have left Hogwarts, actually went to gather reinforcements and led the inhabitants of the town of Hogsmeade and the relatives of the Hogwarts students in a charge that finally sets the tables against the Death Eaters in a Crowning Moment Of Awesome.
There are three other Calvary moments in the same book: The Party Ponies, the Ares cabin and finally Nico, Hades and the army of the dead.
The Party Ponies also act as the cavalry in the second book, The Sea of Monsters.
The Hunters of Artemis fill this role early in the third book, though Thalia in particular is less than appreciative of the rescue.
A Conspiracy of Kings, the most recent book in the Queen's Thief series has an in-universe inversion. Sounis and his army are basically running from the Medes, but are saved by the Attolian army who were just over the hill. Attolis thought Sounis was delaying the Medes, with full knowledge of the waiting backup, but it turns out Sounis was running on blind faith, and had no knowledge of the Attolians. Also a Crowning Moment Of Awesome for Sounis.
Jesus and His army of saints coming from heaven in the Left Behind book Glorious Appearing, which one character refers to as the "Calvary cavalry". However, Jesus does all the fighting with the Word Of God while His army does all the praising.
Animorphs: During the fourth book, the Animorphs save a 'great one' - a whale, though their dolphin morphs think of it as a 'great one', and the whale thinks of them as 'little ones' - from an attack by sharks (and dolphins really do not like sharks, particularly when sharks are attacking a great one). Later on, Visser Three comes after the Animorphs in his own morph (a very, very big sea-going creature from another planet), trying to kill or capture them. The Animorphs send out a cry for help via echolocation. Cue two massive sperm whales, two slightly smaller ones, and the great humpback that they saved from the sharks earlier. Sixty feet long and weighing in at approximately sixty-five tons each, tearing in at absolute top speed to first ram Visser Three's morph, and then subsequently start beating the life out of him with their tails, using blows that are described as being quite capable of knocking entire walls down. The exact same thing happens in book 36, oddly enough, also involving whales.
In the Tom Clancy novel Red Storm Rising the submarines Chicago, Boston and Providence undertake a cruise missile attack on a Soviet bomber base. After the mission the three are jumped by a Soviet submarine which destroys Boston and Providence. The Chicago is saved by the sudden appearance (and the torpedoes) of the British sub Torbay.
Trapped on Draconica: When Team Good is captured and helpess the Eastern Alliance army arrives to save the day.
William Sanders's Journey to Fusang is an alternative history in which Europe lost out to China and the Arabs in the race to colonize the New World. The Big Bad is a Mongol warlord who wants his piece of North America. The hero escapes from the villain and is chased through the desert by Asian horsemen, He is saved by the timely arrival of some Native Americans. Yep, he says it: "If those savages hadn't arrived in the nick of time, those cavalrymen would have killed us."
In the Judge Dee story The Night of the Tiger, the characters need a literal cavalry to save them from bandits, but the road has been cut by a flooded river. Right after The Summation, the cavalry charges in... atop a warship.
In Andre Norton's Storm Over Warlock, Shann is a Throg prisoner, strapped to a frame to enable his being tortured to death. Thorvald and the Wyverns send a mist that traps their minds; then Thorvald arrives in person to free him.
In Of Fear and Faith, August is nearly killed by Fear before he's saved at the last minute by Luthian, who proceeds to kick Fear's ass with his magic and save the day. And he gives August a Cool Sword as a gift to boot!
The Heroes of the Horn are exactly that in the last book of The Wheel of Time. They literally appear out of nowhere and attack the Dark One's forces. They don't actually overwhelm the bad guys, but their surprise appearance and attack is enough to turn a confuse situation into a clear good guys victory.
The Cavalry: The Rifter: Kahlirash’im warrior-priests arriving in Gisa, to the aid of a crowd of townspeople attempting to stop rashan’im soldiers from taking away prisoners accused of witchcraft. Not a Deus ex Machina because the reason for their arrival has been set up in the previous chapter: Hann’yu wrote to everyone he could think of, including the kahlirash’im, about what the Payshmura were doing to prisoners; he’s heard expressing the hope that someone paid attention.
Also, the Rifter, a one-man (or -god) army, breaking the siege of Vundomu.
Live Action TV
Game of Thrones: In "Blackwater," an army on horseback comprised of Tyrell and Lannister bannermen (with Ser Loras Tyrell commanding the vanguard) charge into battle before Stannis Baratheon's forces could breach the walls of King's Landing.
The writers wanted to have a larger force of Klingon ships appear. Unfortunately, they could barely fit those six ships on the screen without zooming out so far that they'd all look too small for a suitably dramatic scene.
The Federation task force in "Sacrifice of Angels" is saved when the Klingons come screaming in from out of the sun.
Martok also decloaked and started shooting right as the Jem'Hadar were about to attack Defiant in "A Call to Arms," when she was constructing the all-important mine field in front of the Wormhole, and thus would have been unable to fight back. Martok played the cavalry a good number of times, actually.
In "What You Leave Behind," it was the Cardassians who saved Defiant when the fleet defected en masse on learning that the Founders were ordering the extermination of Cardassian civilians.
In the Star Trek: Enterprise episode "Zero Hour," Shran shows up in the nick of time and attacks the Xindi superweapon's escort ship—which was about to shoot down the shuttle carrying Archer and the team of commandos who would eventually take down that weapon from within.
In an episode of Star Trek: Voyager, holo-Nazis are bearing down on the heroes. All hope seems lost... That is until some holographic and very drunk Klingons show up!
In Stargate SG-1 the arrival of the Prometheus and its fighters to protect SG-1's defenseless cargo ship in the season 7 finale is a definite cavalry moment.
Bra'tac: They will be in firing range in thirty seconds. More ships approach from the opposite direction.
Carter: Sir! We are about to get our asses kick...!
Bra'tac: They are not Goa'uld...
The Goa'uld system lords do it to SG-1 in an earlier episode, as Ba'al and his fleet come out of hyperspace and immediately begin bombing the crap out of Anubis' mothership before it can make it to orbit.
And the Asgard pull one on Anubis' mooks on his first appearance, Freyr arriving with 2 O'Neill-class cruisers, both outgunning and out-numbering Anubis' Hatak, in an effort to convince them to let SG-1 go.
Hell the first appearance of the Asgard is this, as the audience gets its introduction to Thor's Chariot.
A pretty crap version of this plays out during the Ori invasion arc. A storyline about Teal'c going to get some reinforcements from the local drug dealers, after much angst and wailing, the attack starts, but wait! The drug dealer guys come through. With 3 Hatak (which have been cannon fodder for about 6 seasons), 2 of which get promptly blown out of the sky.
In the Babylon 5 episode, "Severed Dreams," Delenn never looked so beautiful to Capt. John Sheridan as when she swooped out of hyperspace when all seemed hopeless for the besieged space station with four near invincible warships at the last minute to tell the Earth Alliance ships:
Delenn: Only one human captain has ever survived battle with the Minbari fleet. He is behind me. You are in front of me. If you value your lives, be somewhere else.
Sheridan later tells her it was the finest moment of his life. Not to mention that she also managed to deliver the single greatest Crowning Moment Of Awesome in a series full of them, by giving that speech.
Also from Babylon 5 in the episode "Endgame", former President Clark had turned Earth's orbital defense platforms on Earth itself. Sheridan's ship is the only ship within range of one that is about to fire and destroy half the eastern coast of the United States. With weapons out they have to ram it in order to destroy it taking themselves with it. Another Earth ship which had been able to get itself going after being earlier sabotaged by Sheridan's forces arrives and blows the defense platform up.
From the same episode, the battle had initially been between Sheridan's EarthForce ships and the global defense satelites, because he wanted to avoid the appearance of an alien invasion of Earth. Once the satelites begin preparing to fire on Earth itself, Sheridan calls in the Cavalry.
Sheridan: We need you, Delenn.
Delenn: We are there.
Also the season five episode of Babylon 5, A View from the Gallery, has the crew and inhabitants of the titular station holding up against an alien attack until the the cavalry, in the form of the White Star fleet arrives.
The Richard Sharpe adventure, Sharpe's Peril, concluded in just such a fashion, with the British cavalry arriving (led by Barabbas Hakeswill, a character who's loyalty had been ambiguous until now) just in time to break the siege, give Sharpe a chance to defeat the villain and generally save the day.
Averted in the opening episode of Firefly during the Battle of Serenity Valley. The badly outgunned Independent soldiers start celebrating as they hear ships come overhead, thinking their air support's arrived to save them. Except the ships are actually Alliance reinforcements, and the Browncoats are well and truly screwed.
Xander: "Calvary's here. Cavalry's a frightened guy with a rock, but it's here."
The Burn Notice fourth season finale has Michael, Fiona, and Jesse in a half-built hotel, looking to go all Wild Bunch when Sam comes in with the frickin' army.
Band of Brothers.: The Shermans in episode 3. Easy was under heavy attack and Dog and Fox companies had retreated.
Played with in episode 6 where the point is made that Easy Company and the 101st Airborne have never agreed with the story that they were "rescued", as Patton said, by the Third Army.
At the end of the fourth season of Merlin, Arthur's band of loyalists had been whittled down to Merlin and Guinevere. Then the Dragon (somehow) manages to find everyone who escaped Morgana's attack on Camelot and gathers them together at the Sword in the Stone, resulting in a ready-made army of knights and civilians just waiting for Arthur's return. Around the same time, Tristan and Isolde also agreed to fight alongside Arthur.
In one episode of The Saint, Simon Templar actually refers to a useful group of friendly sailors as "the cavalry" after they burst in and beat up the bad guy's Mooks for him.
In the finale of Spartacus: War of the Damned, Agron and Nasir arrive just in time to save Spartacus's life. Well, almost; he's already fatally wounded, but they prevent Crassus from delivering the coup de grace, and ensure he dies free.
Several such events take place in The Goon Show. For example, "Tales of Men's Shirts", which was set in occupied Europe during World War II, ended with the American 5th Cavalry charging in out of nowhere.
Appears in Ace Combat 6, at the end of mission 12. Having destroyed the enemies' nuclear catalyst the Garuda team is ambushed by a huge amount of enemy fighters, after holding out for awhile the entire Emmerian Air Force shows up and saves the day, all while one of the best songs in the game plays in the background.
In Ace Combat 5, mission 17 ends the same way. Huge enemy ambush, Chopper dies, and reinforcements finally show up right about when you run out of missiles.
Ace Combat Zero has the player and his wingman as The Cavalry in a couple battles, particularly the second mission over the Round Table. Allied air units are being shredded by the Belkan Air Force, Galm Team shows up and just eats them alive.
Made more awesome because everytime Galm Team shows up, the Belkan Air Force collectively craps it's pants.
In the 30th scenario of Age of Mythology, you control Arkantos and the others as they try to defend against the overwhelming forces of Gargarensis, Kemsyt, Loki, and some Giant tribes. 20 minutes into the second objective, Odysseus rushes in with a massive army of Centaurs, Hetaroi, Myrmidons, and Hoplites that you use to take down the Big Bad.
This is also played completely straight in a suspiciously similar way in the very first scenario of the campaign, when a lot of heroes rush in to defend Atlantis at the end of the scenario as hordes of pirates land on the beach and you're just getting overwhelmed.
Played straight in Call of Duty 2. After Dog Company takes Pointe du Hoc and destroys the artillery, the Germans launch a counterattack, forcing them to retreat all the way back to the cliffs. When it seems like the final members of Dog Company are about to be overwhelmed, troops and tanks from Omaha Beach finally come to the rescue.
Averted entirely in Call of Duty 4: Modern Warfare. In the last missions, the combined US Marine/British SAS team calls for the "Good" Russians to help save them from the bad guys who are hot on their tail. Unfortunately, The Russians arrive, 5 minutes too late, just in time to possibly save the main character and far too late to save most of the others. However, a Russian helicopter did distract the Big Bad and his henchmen, giving Captain Price an opening to slide his M1911A1 to the main character.
Modern Warfare 2, confirms that Soap and Price were indeed saved by the Russians, in the latter case only to be lost in the system and imprisoned in some hellhole gulag.
In Modern Warfare 3, when Europe was struck by a synchronized chemical attack that has struck every major city in Europe, and the Russians come rolling in. The US come in to aid the fallen European forces.
The opening to Command and Conquer: Tiberian Sun's GDI campaign: Phoenix Base comes under attack by the Brotherhood of Nod; the defending forces are crushed; the last of GDI's infantry scream for reinforcements... and then you enter the fray, commanding an elite group of soldiers dropped straight from orbit.
Darksiders: War summons this at the end of the game and it's quite literal what with them being the other three Horsemen as a sequel hook when told he can't possibly fight the Charred Council alone.
In Dawn of War the Space Marines arived just in time to save the badly beaten Tartarus forces.
In the Tyranid campaign of Dawn of War II: Retribution, the main hive fleet is this; the only reason the player's splinter fleet wants the Big Bad dead is because he's jamming psychic communications. When you contact them in the finale, the hive fleet sends a force big enough to scare the Exterminatus fleet away. OM NOM NOM ensues.
The finale of Dragon Age: Origins has you lead the cavalry, a huge army that you've spent the entire game gathering. You run into Ferelden's capital city to save it after it's attacked by darkspawn.
In Dragon Age II, your reinforcements during the final battle depend entirely on your actions throughout the game. If you managed to hold your team together after siding with either the mages or the templars, whoever isn't in your active party will still join you against Meredith despite the Arbitrary Headcount Limit. Other potential allies include Donnic, Zevran and Nathaniel Howe.
In Final Fantasy IV, the heroes are overwhelmed when the Big Bad's forces revive the Giant of Babel. Then, before they're shot out of the sky, they're joined by the Kingdom of Baron's Red Wings (commandeered by Cid, the Troian Council, and the Mysidian Elder) as well as the Dwarf Kingdom's Tank Brigade. The assembled forces then keep the Giant busy while the party infiltrates it via its throat.
Then it happens again as Zeromus blasts the party into oblivion, and those left behind on Earth send their prayers to the Moon, giving Cecil and his cohorts the strength needed to defeat the enemy.
Made even more heroic as, in the first case, the arrival of the allied forces comes with the Theme Of The Red WingsLeitmotif, also used for the final assault on the The Very Definitely Final Dungeon. In the second, the assembled characters pray with the series' iconic Theme Tune, Theme of Final Fantasy/Prologue as background music.
In an homage to the former event, Final Fantasy IX has the party aboard the Invincible, approaching the portal to the Final Dungeon... only to be surrounded by innumerable Silver Dragons. They're saved at the very last second by the Alexandrian Armada and the Lindblum Fleet, led by Beatrix and Cid, respectively. Despite being rival nations in the past, they proceed to clear a path for the Invincible to break through and reach the portal.
In Final Fantasy XIII your group is cornered and surrounded, and then General Cid Raines of the Air Brigade, known far and wide as 'the Cavalry' comes to your rescue.
Near the end of Ecco the Dolphin : Tides of Time, the rebuilt Asterite sends you to Lunar Bay to assault the Vortex. Not only are your powers from the end of the first game restored, but you're being backed up by a ton of other dolphins who have been summoned by the Asterite.
Heavily subverted in the original Half-Life. The US Marines sent in to quell the alien invasion of Black Mesa are also there to contain all information of the outbreak...by killing every Black Mesa employee they meet, including you.
Reversed in the first Halo at the end of the Silent Cartographer level when the Covenant cavalry falls on you.
In Halo 3, when the Flood crash in Earth and start spreading at the nearest city, the Covenant-Seperatist come in and help contain the Flood.
In Mass Effect 1, you get to call in the cavalry, to potentially save the Council. It is epic.
In Mass Effect 3, The Cavalry can potentially include the Geth, Quarians, Rachni and everyone else you've befriended throughout the series.
Humanity in general seems to love this trope in the Mass Effect universe. It's mentioned that instead of guarding their colonies with ships, they instead strategically place the Fleet at key nexus points in the Relay Network, allowing them to never be more than one jump away should their colonies get attacked. This allows them to easily defend all of them.
This trope can be ironically inverted in Mount & Blade, if the player has both enough cavalry not to be immediately demolished by the enemy formation and enough infantry that their presence or absence can tip the balance. If you're not trying to be particularly strategic and are in a hurry to finish a fight, you may simply charge ahead with your cavalry. But if the enemy turns out to be much of a challenge for horsemen alone, it will be the arrival of your slower infantry playing the role of The Cavalry as far as this trope is concerned.
In Operation Darkness, a late-game mission gives you one of your first glimpses of Germany's Panzer Demons. Without Power Leveling, this fight can whittle you down quite a bit, then more Panzer Demons come, along with some tanks. After a couple turns of desperate music and legitimate fear that you did not bring enough rockets, the American Army charges the field with twice the enemy numbers and much better equipment.
In Parasite Eve 2, there's a point late in the game where our heroine is about to be overrun by a horde of the zombie-ish horrors she has been fighting all game long... only to have the United States Marine Corps arrive with automatic weapons and grenade launchers in full effect.
In the third Ratchet & Clank, the previously cowardly Galactic Rangers pull this off during the penultimate showdown.
In the first London level of Resistance: Fall of Man it looks like suddenly appearing Stalkers are just about to blast Hale into oblivion, but luckily the army comes Just in Time to blow them up and save Hale.
Before the final battle in Skies of Arcadia, every minor character with a battleship shows up at your base in order to provide reinforcements for the final assault.
Star Trek Online has a mission in which you must reclaim Deep Space 9 from a fleet of Jem'Hadar. They put up one last fight to keep the station for themselves, and just when you seem like you're about to lose, the U.S.S. Enterprise-F (Odyssey Class) arrives to help turn the tide.
Played straight with Warcraft III, when Uther brings in a large number of knights to save Arthas in the human survival mission.
Played straight and then horribly, horribly subverted during World of Warcraft Wrath of the Lich King. While the Horde does serve as a cavalry to the Alliance at the entrance to Icecrown, the Lich King's base of operations, once Arthas himself appears and the Horde's own cavalry enters the scene, it was NOT pretty.
Grand Apothecary Putress: Did you think we had forgotten? Did you think we had forgiven? Behold now, the terrible vengeance of the Forsaken! Death to the Scourge! AND DEATH TO THE LIVING!
Villainous example in Advance Wars: Dual Strike. The fourth mission in the game give you just 7 turns to beat Lash before Flak and his troops arrive and curb-stomp you right into an instant game over.
In Saints Row The Third, purchasing the "Saints Backup" upgrade gives a chance that Saints redshirts will show up to help you after you get into a fight.
Towards the end of Mother3 the main characters are being swarmed by a never ending army Mooks and slowly being worn down until the DCMC bust in and start taking them down.
Tears to Tiara 2: The Goddess Ashtarte summons one in the form of Elven elephants and Taros.
Gunnerkrigg Court: After an Our Hero Is Dead cliffhanger in Chapter 7, Chapter 8 begins with the TicTocs swooping in to break Annie's fall. Interestingly this is the first (and to this date, only) time we see the TicTocs in a group, actually doing anything.
In Goblins this happens during the battle in Brassmoon City. Goblinslayer's men is about to overthrow our heroes fighting in the Battle Of Wonder, when Thaco arrives with all the prisoners escaped. Cue Oh Crap by Goblinslayer.
In the Phoenix Wright-inspired Kongregate game "Socrates Jones: Pro Philosopher", the title character is trapped in the Intelligible Realm aka Philosopher Heaven and has debates with five famous philosophers's ghosts in an attempt to discover the nature of morality so that he and his daughter can return to life. In the process, Socrates gets each of them to question their own philosophies. Near the end, he goes up against the Arbiter, overseer of the realm, after an off-hand comment is mistaken for a bad answer. Despite Socrates' efforts, the Arbiter refuses to accept his expanded answer, and Socrates is ready to give up. But before he can do so, the philosophers he debated with earlier show up to support him, and four of them give Socrates an idea from their respective debates that he can use to challenge the Arbiter's arguments.
In Greek Ninja, Sasha is saved from the advances of a succubus/ex-lover by the wandering demigoddess Electra.
Look to the West: Heinz Kautzman's Russo-Lithuanian-Danish-Courlandish force in the Battle of Paris. Subverted in that they were not needed, but were only there so those previously mentioned countries could claim participation in the war and find themselves part of the peace talks. It works.
The arrival of the Reds (and Tucker, and Caboose) in episode 19 of Red vs. Blue: Revelation.
Wash: I would say that was the cavalry, but... I've never seen a line of horses crash into the battlefield from outer space before.
In the Whateley Universe novel "Ayla and the Birthday Brawl", when Chaka and Riptide have been taken down by The Lamplighter, they are saved by... a team of Knights of Purity in power armor. Chaka hates the Knights of Purity.
At the end of The Adventures Of The League of S.T.E.A.M.'s season 2 finale, "Dead End", The rest of the League come to the rescue when Crackitus, the Baron, Katherine and Thaddeus get outnumbered by zombies.
Season 3 finale of Noob has their faction's elite take over after spending some time trying to keep much stronger enemies from reaching their intended prisonner.
Justice League, "The Savage Time". Hawkgirl is in trouble with three Me109s on her tail shooting at her accurately enough that she has taken a bullet through her wing. Suddenly more tracer fire comes in from offscreen and through the Luftwaffe planes. Cue dramatic music as the Blackhawks swoop in.
And then later, The Flash shows up, and he's brought the U.S. Navy with him!
Before Madame Rouge has a chance to squash Beast Boy like a bug in the second-to-last episode of Teen Titans, the ground beneath her starts to crumble... out pops Cyborg and some reinforcements! Ceiling bursts open... and Starfire has some more! Finally Raven brings in the last of them and Battle Royal against the Brotherhood of Evil begins!!!
Jonny Quest TOS episode "A Small Matter of Pygmies". Jonny, Race and Hadji are trapped on a hilltop with armed angry Pygmies closing in from all sides. Suddenly in the distance they see a flight of helicopters approaching, with Dr. Quest as a passenger in one of them. The helicopters blow away and otherwise roust the pygmies.
In the season finale "What Lies Above" part two, this works out much better for Lion-O. Just when things look especially bleak with The Reveal of Pumyra's true allegiance to Mumm-Ra and Mumm-Ra having defeated the Thundercats with the Tech Stone's power the Thunderkittens show up with a small yet formidable team composed of the various Animals Lion-O has aided during the series. The Fishmen, the Elephants, the Ro-Bear Berbils, and the Dogs launch an all-out attack that catches Mumm-Ra and Pumyra off guard, forcing them to retreat.
Back in the middle ages the cavalry, the horsemen, usually come in after the foot soldiers. After the guys in the front weaken the first waves, the cavalry come in to deal more damage.
Jan Sobieski and the Polish cavalry rescuing Vienna. Yes, they really were literal cavalry, and numbering twenty thousand, they were the largest cavalry charge in history.
Poland in general likes its horses. How much? They still use a cavalry unit (though it's mostly for parades and occasional border patrol). Likewise many countries with official "cavalry" units retain ceremonial horse cavalry units. US 1st Cavalry Division, for example, still has a horse mounted unit (albeit used almost exclusively for ceremonial duties).
During the First Crusade the Christian (Frankish) army, huge for the time, divided into two columns to pass through Anatolia. The Muslim (Turkish) army attacked and encircled one Crusader force, and appeared likely to defeat it, when the other Crusader force showed up unexpectedly. Given that knights were rescuing knights, this would be a case of the Cavalry rescuing the (dismounted) Cavalry; since the Turks were almost all mounted archers, that would be...
Marshal Blücher and his Prussian cavalry (and infantry and artillery) rescuing Wellington's force at Waterloo.
The famous World War II battle with the Destroyer, USS Laffey, which was being continually hit with kamikaze and conventional attacks while it was all alone doing radar picket duty, two cavalries came into play. When things were looking bad, four US fighters from the escort carrier Shamrock Bay arrived to shoot down as many as they could, then buzz the enemy planes when they ran out of ammo, until they were forced to disengage due to low fuel. When all seemed lost for the Laffey, a second cavalry arrived in the form of a dozen US F 4 U Corsair fighters to take care of the rest of the enemy.
The Coaltion force in Libya was this to the Rebel forces in Benghazi, with hostile forces closing in and enemy tanks at the entrance to the city one rebel said "So, we’re being abandoned after all." Enter the Frenchairstrikes and later the 120+Tomahawkmissiles.
Subverted initially in the American Civil War by Union cavalry, who had the reputation of retreating whenever they were met with any kind of resistance. An old joke among soldiers, "You ever seen a dead cavalryman?"
Ironically, one of the most famous cases of this trope concerning the Federal cavalry involved Buford's cavalry division being saved by infantry of the Army of the Potomac's I Corps, while delaying Lee's advance on the first day of the Battle of Gettysburg.
More in the minor-character-rescue vein, the Dunkirk evacuation: while it may have been a Curbstomp Cushion for the general public, for the French and British stuck on the beaches facing the Wehrmacht with their backs to the sea, the arrival of hundreds of ships of every variety to rescue them in the nick of time was definitely the real thing.
The British Commandos during the D-Day invasion, who relieved D coy, 2nd Ox and Bucks Light Infantry at the aptly named Pegasus Bridge (given the cavalry reference that is—the bridge was renamed after D-Day for the flying horse emblem of the British Airborne).
Another World War II example, depending on who you ask: Patton's breakthrough at the Battle of the Ardennes (the Battle of the Bulge) is widely credited with rescuing the trapped 101st Airborne Division dug in around Bastogne. Of course, you'll get an argument from the 101st that they never needed rescuing in the first place...
A subverted example during the Battle off Samar: A large Japanese surface group consisting of the giant battleship Yamato and several cruisers launched a surprise attack against the American landing group in Leyte Gulf. Opposing them were two destroyers (Johnston and Hoel) and two destroyer escorts (Samuel B. Roberts and Heermann). The crews of the destroyers only needed to be told once: "small boys attack." At which point they fought the Japanese task force to a standstill. The first wave of reinforcements received by the severely out-gunned Americans was in the form of Navy fighters and light bombers launched by the escort carriers the destroyers were desperately fighting to protect. Meanwhile, Halsey's fleet was alerted to the danger and turned to rush to their aid. The cavalry eventually did arrive, but too late to affect the outcome. In the end, the largest battleship in history (displacement and main armament, the American Iowa class were longer was driven off by a handful of destroyers.