There's a big bad gang/army out there that wants to sack the peaceful village for whatever reason. The reason the heroes don't just smite the villains is usually that there are too many of them for the heroes to deal with; although this may conflict with their previous One-Man Army exploits.
Either way, it's time to teach the villagers to stop being victims and learn how to defend themselves. Expect some complaining about "losing their peaceful way of life." If there's a Wasteland Elder around, expect them to back the hero and shush the young'in.
The villagers will fight back with pitchforks, plowshares and frying pans. It will also include many Bamboo Technology traps involving previously innocuous items, like laundry buckets filled with boiling water. Usually, the outlaw band/army should have been able to beat them, had they taken the villagers seriously and fought with actual strategy; but their downfall usually lay in rushing into traps.
In some instances, the villagers may have existed under the oppression of the Mongol Hordes for years or even generations without even thinking of fighting back. Not, that is, until The Hero shows up. Bathing in the glow of his shining example, the common people finally rise up to combat the Evil Overlords.
With the enemy dead or on the run, and the villagers converted to the notion of violence for self-defense, the heroes leave.
See also The Magnificent Seven Samurai, The Paragon, Perfect Pacifist People, Home Guard, Instant Militia. The village is often a Doomed Hometown that fights back. Compare Defiant Stone Throw.
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Anime and Manga
Cyborg 009 uses the trope and ends the episode with a subtle but ominous picture of one of the formerly peaceful (not to mention god-like powerful) aliens callously trampling over a flower.
Subverted in Naruto: pretty much everybody in the villages are extremely deadly ninjas to begin with.
Done darkly in an early Groo The Wanderer comic. Groo trains a peaceful village to fight against two hordes of bandits. In the process, the village and all it's crops are destroyed. Having no other way to make a living now, the villagers and the bandit hordes unite into a single, massive bandit army that overruns nearby cities.
Darkly and surprisingly often- another, even earlier story had women being kidnapped by an airship. Groo 'trains' the villagers (with the usual complaints of the attack having been less painful and going into battle being safer than Groo's training). Turns out that the women were being held in a... basically a resort... and had no intention of leaving, and that they rather resented being 'rescued'. A later story involved bandits riding giant birds, and while Groo's training more or less worked, his help, as usual, backfired spectacularly.
ElfQuest: When their revered Mother of Memory is entrapped on the spirit plane by an enemy they can't begin to comprehend, the peaceful Sun Folk ask the few remaining Wolfriders to teach them weapon skills. No, they never use their skills in any useful way during the main storyline, but at least learning to shoot arrows makes them feel like they're doing something.
The issue was more that the Wolfriders were leaving, and since the arrival of the Wolfriders had precipitated the departure of Rayek, their previous warrior-defender, they felt that they would have no more protection. Plus their healer was leaving them at the same time.
Shamelessly parodied in a Thrud the Barbarian story. The Xena-eat-your-heart-out-gorgeous warrior woman Lymara shows the village women how to kill two men. She drops her sword and bends over to get it, giving the men a great view of her cleavage. The two men stop and leer. She swiftly beheads them both with a cry of "Hah! Sexist pigs!" Then she looks at the (realistic) peasant women for a moment and asks, "On second thoughts, who knows how to shoot a bow and arrow?"
Done in an issue of Marvel Adventures: Avengers, where the Avengers have to train a village to fight off a warlord and his minions. The Avengers fight along with the villagers in versions of their costumes modified to look as if they're villagers themselves, to try and make sure that the warlord won't just regroup and come back the moment he thinks the Avengers have left.
Even more, Cap couldn't train them because it would tip the warlord off - he had Wolverine train them. It was like a dream come true to him.
Done in a silly manner in the movie ¡Three Amigos!. The villagers couldn't do much except dig and sew, which was a problem since the heroes couldn't do much except pretend to be cowboys in silent movies. But with a plan cobbled together from the big finales of their various movies, a Rousing Speech that got a bit lost in the middle, and a lot of digging and sewing by the villagers, the day was saved.
The Spaghetti WesternThey Call Me Trinity has the heroes teaching a community of pacifists to fight back against the villain who wants the land they've built on. The villagers are so peaceful that they have no idea how to fight, and the heroes end up repeatedly clobbering them during the training exercises.
The Ant Colony from A Bug's Life. Even the grasshoppers recognize the Ants' potential threat (They outnumber the grasshoppers, 100 to 1), but the Ants don't... until it is expressly spelled out for them.
The whole premise in Defiance, which is about a group of Jewish refugees. As they have gathered more people, they begin training able bodied people (including women) to fight for survival with weapons taken from the hands of dead Nazis.
In Army of Darkness, Ash is inexplicably seen teaching the medieval soldiers how to use a halberd.
Subverted in Ip Man, where our hero trains the workers at Quan's factory in Wing Chun to help them resist a group of bandits, only for the bandits to prevail anyway until Ip Man pulls a Big Damn Heroes moment.
This is how they deal with the bad guys in Dragonheart. The villagers weren't exactly peaceful, however; rather, they were worn out from trying to fight the current Big Bad's father some ten years earlier, and just didn't want to get involved again. Having a dragon on their side changed their minds.
...and then outright parodied in Robin Hood: Men in Tights, where the Peaceful Villagers are so inept that Robin and company have to do pretty much everything themselves.
In The36th Chamber Of Shaolin, the eponymous "36th Chamber" of the hero's training is to find and train commoners to retaliate against an evil regime of some kind. Although he pretty much does all the killing himself, it's an interesting coda to the most protracted Training Montage in martial arts cinema.
Subverted in High Noon, where Gary Cooper plays a marshall who hears that a criminal gang is coming to his Wild West town bent on revenge. Despite the fact that he was already planning on leaving, he spends most of the movie attempting to rally the villagers to the defense of their town. Everyone else proves too cowardly to fight, however, and he is forced to take on the gang almost singlehandedly.
Cave Dwellers has Ator ride into a village and immediately start this sequence (seriously, he begins the "This is what we'll do" speech before he even gets off his horse)... only to get pissed off on realizing the villagers aren't playing along. Turns out they're not on his side.
Done to a degree and lampshaded in Kingdom of Heaven. After Balian knights all of the men in the village as a group, the priest asks if making a man a knight will make him a better fighter, to which Balian simply responds "yes."
This is the plot of one of the Animorphs books, where the peaceful villagers and the threatening invaders are both aliens. Also joining the fight are a group of Muggles who are helping to resist the invasion simply because they Jumped at the Call. They're all Trekkies on their yearly camp out.
It was also the plot of The Hork-Bajir Chronicles. An Andalite trains the Hork-Bajir to fight against the invading Yeerks. This was necessary because the Hork-Bajir were bark-eaters, and had never had conflict before. They, at first, couldn't even conceive of using their blades to deliberately harm another.
The Redwall series takes this to ridiculous extremes. The villagers are frequently absurdly outnumbered, pacifists who have never fought a day in their lives, and the villains bother to use actual strategy. (Although to be honest, all but the small handful of leader-types in any given vermin horde tend to be hopelessly inept redshirts in their own right.)
Defenders' Advantage—it's commonly accepted that to take a walled city or castle (which Redwall Abbey pretty much is) using medieval weaponry, the attackers need to outnumber the defenders by at least ten to one.
Too bad the strategy never seems to extend to using siege weaponry to knock a wall down and rebuild it. The plans are still around, aren't they?
To give an actual trope example: in "Mariel of Redwall," a group of sea rats are besieging Redwall Abbey, as they do. The searats have just hit on a strategy to get inside that may actually work — fling balls of burning pitch over the walls to burn them out — when a Long Patrol of three hares from Salamandastron arrive, and teach the Redwallers the construction and use of longbows.
In the animated adaptation this often turns comical, with one attempt to tunnel into the Abbey repulsed with a vat of hot oatmeal. In the original book, it was boiling water.
Used in The Sworn Sword when Dunk's current master gets into a border dispute with a much more powerful neighbour. Dunk sees the futility in pitting their poorly trained peasant conscripts against a larger force of professional men-at-arms. Ultimately a bloodbath is averted.
Kelsier does this in the first book of Brandon Sanderson's Mistborn trilogy.
Subverted in The Wheel of Time with the Tuatha'an (an ethnic Expy of the Roma), who follow a pacifistic moral code known as, "The Way of the Leaf". The Tuatha'an refuse to defend themselves even against what are essentially the Hordes of Hell, much to the anguish of Perrin and Aram, a Tuatha'an who becomes "Lost" and learns to fight when his parents are killed by the Trollocs. The Aiel have a mass BSOD when they discover that their ancestors were this.
However, played straight when Perrin rallies the people of Two Rivers to defend themselves against first the Trollocs, and then the Children of the Light.
Of course, the Two Rivers folk turn out to be pretty skilled already, being some of the best archers in the world and having the blood of Manetheren, which counts for a lot in Randland.
In Alan Quartermain, when a Masaii warband kidnaps the daughter of a missionary, the Zulu warrior Umslopogaas makes the plan for the parishioners to attack and annihilate the war party.
In Barbara Hambly's The Ladies Of Mandrigyn, the eponymous, and previously very traditional, women secure the services of mercenary captain Sun Wolf to train them in weapons use so that they can free their men from imprisonment in the local mines and help to overthrow a vicious wizard/warlord.
In the Ciaphas Cain novel Death or Glory, Cain creates a small army by having the PDF troops he had gathered train volunteers among the civilian refugees to supplement their ranks. The popular histories of that war claim that the bulk of the army was made of such volunteers (which was most definitely not true). In Cain's Last Stand, Cain realizes that the understrength PDF on the same world doesn't have the manpower to repulse the incoming Chaos fleet, so he creates militia units across the planet using this trope. Subverted in that he had no illusions about the ability of the militia to repulse the invaders: He was simply hoping that they could pin enemy units down long enough for properly trained soldiers to show up (and as the retired Sister says, if the war was lost they'd be slaughtered anyways by Chaos cultists).
Vanyel tries to do this in the first book of the Last Herald-Mage trilogy; he realizes almost immediately that he just doesn't have time to train them in new weapons, and focuses on stratagems involving things they already know how to use, like pitchforks.
In the second book, Eldest, of the Inheritance Cycle, Roran does something similar in Carvahall.
In the alternate history novel The Years Of Rice And Salt, a samurai from Chinese-conquered Japan makes his home among a native American tribe. He teaches them the arts of warfare and modern agriculture and industry so that they will be able to withstand the advances of the Chinese colonisers on the West Coast (and the Islamic colonisers on the East Coast). Centuries later, the various native American tribes had formed the Hodenosaunee League, which survives as a major world power.
In The Dresden Files, the ParaNet is organized to protect minor talents from being abused by black wizards or other supernatural threats.
It's more successful than anyone expected, and becomes necessary for survival after Harry dies.
The most controversial book of the Sword of Truth has a pacifist MacGuffin people called the Bandakar Empire, that Richard and Co. train to fight.
This trope shows up in the George R. R. Martin short story "And Seven Times Never Kill Man". The fanatical Steel Angels create a colony on the world of Corlos, and begin systematically wiping out the native, simian jaenshi whenever they feel the need to expand their territory. The overweight, amiable, and formerly pacifistic trader Arik neKrol takes it upon himself to organize a jaenshi resistance. The story subverts the trope. Very few of the jaenshi gravitate to his plan, all are half-wild survivors of previous Steel Angel attacks on villages who did not join other existing clans, only one of the group has any kind of discipline, and Arik is killed in their first and only skirmish. But then it's double-subverted, because it turns out that the jaenshi didn't really need an armed resistance at all. They apparently concoct a ruse (the how of which is deliberately left vague) using the mysterious pyramids their villages are based around, that plays on the Steel Angel's fanatical religious beliefs. The only thing Arik's resistance ends up accomplishing is killing the one Steel Angel officer that suspected the jaenshi were somehow deceiving them.
The Star Trek: Enterprise episode "Marauders" has the Enterprise crew do this with a mining colony that was being raided by a group of Klingons.
Combined with a Stable Time Loop in a particularly bizarre episode of Kung Fu: The Legend Continues. A time-displaced Peter is surprised to learn the besieged group of Shaolin monks he's landed among have no clue how to fight off their attackers. The rudimentary self-defense he teaches them eventually became the basis for the kung fu his Shaolin father taught him.
Rimmer tried this with a group of entertainers and pacifists in "Meltdown," and expended the few troops that survived his training on a suicide mission.
An episode of Merlin uses this trope after the title character's home village is attacked by raiders.
Doctor Who did this in the original Dalek story, only rather than being ignorant the Thalls where instead morally opposed to violence due to the wars of their past.
Star Trek: The Original Series subverts this in "Errand of Mercy," when war breaks out and Kirk and Spock try to get the apparently primitive citizens of the planet Organia to side with the Federation. However, the Organians aren't interested and seemingly go along with the Klingons seizing control of the planet while resolutely protecting the Starfleet officers, who are trying to rouse a resistance, with incomprehensible motives and means. In the end, the Organians suddenly stop the war themselves and reveal that they are actually energy beings of godlike power and were only humoring the warring powers around until their patience ran out.
Played straighter in "A Private Little War" where Kirk must decide whether to arm and train the Hill People of a planet who are being attacked by previously peaceful Town People (and to make matters worse, he had surveyed the planet beforehand and found it a peaceful paradise, as well as befriended the Hill People personally). It turns out the Town People were being given lessons in weapons development by the Klingons, allowing Kirk the legal pretext to arm the Hill People, while tortuously aware that he is starting an arms race that will plague the planet. It's an allegory to the then-ongoing Vietnam War.
In "The Omega Glory", Kirk has to stop a fellow officer from arming the villagers of the last village left on the planet, who all look Chinese with phasers against wild, brutish invading Americans who worship freedom but are pretty fuzzy on its finer points. This one is also a Cold War fable.
Subverted in Robin Hood. In an early S3 episode Robin rallies the villagers and tells them that one day he will call upon them to fight; likewise a press release revealed that Tuck would be organizing a "people's army." Turns out that what Robin and Tuck had in mind was for the villagers to stage a peaceful sit-in protest.
One episode of Firefly ("Heart of Gold") involved the crew digging in to help train and defend the local... whorehouse, complete with montage. It works.
Pretty much the backbone of the plot of Buffy's series 7, with the primary focus being Buffy training potential slayers to be as close to her abilities as they can without the actual supernatural element having taken effect yet. This is in order to defeat not just an evil force or being, but, literally, the very concept of evil itself, in the immortal guise of the first evil to ever enter existence. Granted, each one of them is potentially the most deadly woman on the planet in any fight that doesn't involve weapons of mass destruction being deployed, but until Buffy decides to go against the thousands of years of established, supposedly irrevocable tradition they're just ordinary girls.
...which makes their training at least five times harder than almost any other example on this page. With, literally, their lives at stake, pointing all the potentials in a single direction for more than one episode is like herding cats. Much of the season's drama comes from many of the girls choosing to follow Faith's example, others Kennedy's. Most follow Buffy at first, until she begins proving unequal to the task and rashly leads them into an ambush, killing several. With all the Lord of the Flies-type bickering going on, the only real training shown on screen is Kennedy leading the potentials through a martial arts kata in the back yard, yelling at one girl who screws up like a Drill Sergeant Nasty.
Stargate SG-1 tries to do this with a planet about to be invaded by the Ori. They only have time for a quick crash-course in using P90s, which does not include reloading. As soon as ammo runs out, the villagers surrender.
Stargate also has a villainous example with the Ori followers. Aside from the whole burning people thing, the Ori army were previously decent people. The Ori whipped them into an effective army rather quickly.
The outlaws in Robin of Sherwood travel to the village of Uffcombe in a two part adventure, where the villagers are being terrorised by the Hounds of Lucifer not supernatural demons as it turns out but merely the horseriding, mask-wearing minions of the local Priestess of a Satanic coven which is pretending to be a religious order of nuns. No, really. Actually...it works.
An episode of Legend of the Seekerzig-zags this. On finding a peaceful village being harassed by raiders, Richard tries to train volunteers to fight in defense, only to find that they're under an old spell that paralyzes them should they even try to be violent. Richard convinces Zedd to remove the spell despite his objections, and finds out why it'd been cast in the first place: The villagers are the descendants of a group of brutal enforcers for an ancient ancestor of Richard's. Their anger is linked to him, and through him to the Sword of Truth, fanning their rage. This leads to them attacking and killing some of the raiders in their sleep, prompting the rest of the raiders to move for revenge—leaving Richard with only himself and a handful of men freed from the spell against the hundreds of raiders. They set up a bluff, arming the rest of the village, and try and convince the raiders to leave...which only works because one of the (still peaceful) villagers had saved one of the raiders earlier in the episode.
An Andromeda episode has Dylan arrive to train a tribe of peaceful locals to fight off-world pirates armed with guns. Dylan brings a crate-load of force lances (extendable pikes that double as guns with guided ammo), but a religious fanatic destroys them, refusing to allow Dylan to corrupt the locals. Instead, Dylan has them build fortifications and trains them to throw spears from the walls. They win.
This is in the backstory of Mortarion, Primarch of the Death Guard Space Marine legion in Warhammer 40000. He was adopted by one of the overlords of the planet he was found on, but later escaped into the valleys, where the air was breathable for normal humans, and trained them to resist the overlords (who used them as slaves and experimental subjects), forming the Death Guard (not originally Marines, but the legion took the name) to fight. The Salamanders' Primarch pulled off a similar trick, beating Eldar raiders around the head with his blacksmithing hammers.
Also a tactic sometimes used by the Space Wolves, as described in the codex. An example has a squad of grey hunters being ordered to evacuate the main spaceport to allow orbital bombardment of it to prevent it from falling into enemy hands. The Space Wolves, in their rebellious natures, instead decided to stay and not evacuate the spaceport, to train the local populace until the enemy arrived so they could defend the spaceport instead of losing it forever.
In Traveller, mercenaries are often hired to do this.
Frighteningly prevalent with experienced players in D&D.
The degree to which this can work varies by edition. As of 4E, you can arm everyone in town with reasonable gear and give them weeks of training, but they'll still have exactly one hit point and die from the indirect side-effects of a single enemy's attacks. Third edition let them live long enough to buy the heroes time and/or provide flanking bonuses. Second edition would let them win almost any fight hands-down through mob rules for overbearing.
Once used to defend a village against a rampaging horde of kythons. This was not the worst thing in the campaign.
Though perhaps more frightening when done in Exalted. Tiger Warrior Training can turn even the smallest peasant villager into a lethal combatant.
Also, one of the secondary signature Zenith Caste Solars, Karal Fire Orchid, does this in her origin story.
Infernals can't train them anywhere near as well (yet), but can provide the peaceful villagers with power-enhancing mutations to make them just as dangerous. If you really want an unholy terror, get a mixed Circle to use multiple Charms of this sort on the same group of villagers, then hold off the monsters with firebreathing, taloned Super Soldiers.
Happens in one of the Pathfinder Society modules taking place in the Asian-inspired continent. A village holding the Lost Heir to an artifact's power is annually attacked by bandits at harvest time. It is up to the PCs to train the villagers to defend themselves in the upcoming battle.
A lot of strategy games start out this way, usually as the tutorial.
You had to do this in Jagged Alliance 2 if you wanted to keep Deidranna from re-taking any liberated towns. the unofficial v1.13 update makes them a lot more dangerous.
In Pikmin, the titular creatures are nigh-extinct until Captain Olimar assembles them and exploits their sheer numbers and abilities. When he takes off, the Pikmin start fending for themselves. By the time the sequel came around, they are found to be, while not numerous, in much better shape than before, managing to survive on their own.
In Roadwar 2000, your gang can recruit the starving "Needy" people, who are hands down the most worthless of all the post-apocalypse city wanderers who can possibly join you... but with a good drill sergeant, you can promote these recruits into worthy gang members after only a few firefights or road battles to harden them.
The western chapter of Live A Live mostly features Sunset (the hero) and his rival/partner Maddg searching the town for items to give to the peaceful townfolks to use as traps (ranging from dynamite and ropes, to horse droppings, a pachinko machine, and a naughty poster), in order to decimate the villain gang.
In Fallout 3, you can help the population of Big Town defend themselves from Super Mutants with guns, robots, mines, or even just hiding inside depending on what skills you have. However, you may find it more amusing to leave them to die.
Deconstructed in the Fallout New Vegas DLC Honest Hearts, where the player must choose to help a pacifist tribe called the Sorrows, either by teaching them to fight back or help them escape and settle elsewhere. Daniel, the Mormon missionary who advocates resettlement, worries that teaching them to fight will lead to the loss of the Sorrows' innocence and calls you out if you choose to do so. If you help them by fighting, the epilogue reveals they end up becoming aggressive Proud Warrior Race Guys that start waging wars with other tribes.
It's also played straight in the early quest "Ghost Town Gunfight" where you form a impromptu militia of Goodsprings townsfolk to fight Joe Cobb and his small group of Powdergangers. The best thing you can do is get Trudy to round up the townsfolk, get dynamite from Easy Pete, armor from Chet, and meds from Doc Mitchels. This allows the town to beat the gang.
In Mount & Blade, villages may ask you to help teach them how to fight better to fend off bandit attacks. You had to drill them, beat up several of the recruits and then fight off an attack after completion. Their relation with you improves as a result.
The opinion of their respective lords, however, worsens.
The entire point of Battle Realms. Unlike most other Real Time Strategy games, you can not actually purchase new units (aside from Heroes). Instead, you train the worker units (aka Peasants) into different schools to get the units you want. Training them again in one of the remaining schools mixes their education and makes them into different units; and so on.
The village of Redcliffe in Dragon Age: Origins is besieged by The Undead. You can choose to help the villagers defend themselves by convincing their drunk-off-his-ass blacksmith to supply them with new armor and weapons, persuade/coerce the somewhat shady dwarven warrior Dwyn and his thugs to fight, offer an Elvish archer spying on Redcliffe for Loghain mercy in exchange for his help, convince Lloyd the bartender through threat or persuasion to offer the militia free ale and/or make him fight too, find oil that can be used to set a trap at the village entrance, and provide the Knights of Redcliffe with amulets from the local Chantry that help boost their morale. Of course even with all of this, they still won't stand a chance without you and your party fighting alongside them. If you don't want to go to all of this trouble, you can also just leave them to the hungry zombie horde.
Human players in Warcraft III can convert their peasants into milita in an emergency. They turn back into normal peasants after a while, though.
If Shepard has the 'War Hero' background in Mass Effect, s/he (at 22 years old) rallied the civilian colonists of Elysium against a slaver attack. When the defenses fell, Shepard single-handedly held a position for several hours until reinforcements arrived.
Mirror!Gwynn: If we must resort to brutality and violence to survive, then we have already lost!
Torg: Then go. Be lost.
Also subverted in Tales of the Questor where the town of Freedom Downs has a Farmer's Militia for self defense and if Quentyn is any indication, the standard training he got long before he decided on his calling was damn good stuff.
One episode of Samurai Jack had Jack teaching a group of peaceful monkey-men to defend themselves from a thieving, bullying rival tribe, in exchange for their giving him some training to enhance his jumping abilities.
Bounty Hamster. Parodied in "The Good, the Bad, and the Adorable", most notably in the scene where the cute alien villagers decide that the best solution is to steal their saviour's spaceship and run away en masse.
The first episode created for Wakfu (chronologically the fifth episode) has this as a premise, the heroes trying to teach a peaceful society of blob-people to stand up against the bull-creatures who are stealing from them.
In Star Wars: The Clone Wars, Obi Wan, Anakin, and Ahsoka, along with a few bounty hunters, teach the farmers of a small village to fight pirates and protect their homes.
The ThunderCats (2011) episode "Berbils" has Panthro and the Cats' teach Ro-Bear Bill and the cute and cuddly Ro-Bear Berbils how to defend themselves form those who would use them as slaves.
Truth in Television, to an extent. Certain special forces groups worldwide, including the British SAS and the US Army's Green Berets, are sometimes tasked with raising up rag-tag revolutionary armies to bring down governments perceived as exploitative or antagonistic. The peacefulness of the villagers in question, however, varies wildly.
Afghanistan is the key example most folks will use. The villagers were never peaceful, but their pre-1900 rifles didn't do squat to attack helicopters. US trainers and assistance came in and gave them Stinger missiles, and not long after the Soviets learned how America felt in Vietnam.
The Taliban then used this against the Coalition forces during the War On Terror. Although in this case, "training" consisted of giving local farmers a wad of money and a cell phone and telling them to call the only phone number in the directory when a Coalition convoy passed by the nearby road. And that's how you got the many, many ISAF deaths.
Ironically, this happened in Vietnam too. There was a group of tribes collectively known as the Montagnards, who were routinely harassed by the North and South Vietnamese. US Special Forces provided them with training and some supplies and tools, and in return they ended up proving to be very resourceful allies on the battlefield, who provided American forces with LOTS of helpful information. Too bad we didn't use that knowledge to make our lives easier in the past two decades' conflicts.
America itself only had a small standing army between 1865 and 1916, and especially between 1920 and 1940. Even in the latter case, the ability to raise an army as quickly as they did surprised a lot of other countries' leadership, on both sides.
Of course there is the example of the Prussian officer Baron von Steuben who, during the Revolutionary War, taught the ill-trained American Contintental Army how to fight as well as the British.
The Semai people had no violence, or even any words for violence. When they were taught to fight when the area became a war zone, they surprised their "teachers" by becoming frenzied and intoxicated with the bloodshed, since they didn't fully understand the effects of their actions. They are still extant, and returned to their non-violent ways following the conflict ending.
A key job of the British Special Operations Executive and the American Office of Strategic Services during World War II was to organise and train operatives to go behind enemy lines and train / supply resistance groups. Given the nature of the job, it was often a one-way journey.
Ninjas were just farmers who were oppressed by the samurai. They then trained themselves to fight back with their farm equipment (Kunai and such).