Evil Only Has to Win Once

"Spike: We just keep coming. But you can kill a hundred, a thousand, a thousand thousand and the armies of Hell besides, and all we need... is for one of us, just one, sooner or later, to have the thing we're all hoping for.

Buffy: And that would be what?

Spike: One good day."

- Buffy the Vampire Slayer, "Fool for Love"

As Long As There Is Evil, good must rise to the challenge and defend the world. Beating the Big Bad and his pawns will maintain the Balance Between Good and Evil and keep the world safe. Good doesn't always win however, whether it's Stage One of the Evil Plan or in the final battle, heroes do occasionally lose. In these cases the old heroes, or a group of new ones, must again rise to challenge the bad guys, usually with better results.

In some stories this won't happen, because The World Is Always Doomed. Evil only has to win once in order to permanently turn the world into a crapsack Villain World with absolutely no hope of being deposed. Heck, if the villain is an Omnicidal Maniac there won't even be a world to save.

When Evil Only Has To Win Once, heroes need to step up their game because the Sorting Algorithm of Evil just divided by zero. Usually this shows up in stories where there is some kind of repeating challenge against the forces of good; be it a martial arts tournament for control of the planet, a resurfacing Sealed Evil in A Can, or just a laundry list of progressively more dangerous enemies. You can expect The Chosen One to feel the pressure — there are hundreds of mooks, dozens of mid-level bad guys, a Dragon, one Big Bad villain — and one Hero (or a team, but you get the idea) to face the threat. There's no one else to face the threat... usually, at least. Hopefully the replacements can do the impossible.

Either Or Prophesies use this trope with language along the lines of "If Dark Lord Genericide finds the Ruby Heart he will bring about a thousand years of sorrow!" Rarely, the prophecy's "Or" has the heroic variant included: "... but if Sir Tropesalot finds the Sword of Plot Advancement, he will take up the crown and usher in a thousand years of peace!"

It just doesn't seem fair. Why is it that beating the Big Bad doesn't bring about a thousand years of perpetual (hopefully not literal) light? There's two potential justifications for this: Good, true Good, won't stoop to the levels necessary to create a permanent Sugar Bowl, whereas Evil won't have a problem with completely wrecking the world. The other reason is that the Good Guys are usually the only ones concerned with maintaining the Balance Between Good and Evil. Metatextually, the Author also probably wants a world where his Chronic Hero Syndrome suffering protagonist can't sleep. Also, we know Villains Act Heroes React, and an action can often be tried many times until successful.

This trope is often the unsaid justification for having Showy Invincible Heroes.

True in pretty much all video games. You have to win through every challenge you face to get to the end, whereas your enemies just have to win once and (unless it's a Hopeless Boss Fight or Final Boss Preview) it's Game Over. It's also frequently inverted, from the perspective of a player stuck against That One Level or That One Boss; no matter how many times you get beaten down, you (generally) only need to win once in order to progress.

Anime & Manga

 * In Neon Genesis Evangelion, if even one of the Angels penetrate to the heart of the NERV base, Third Impact will be initiated, wiping out all life on earth.
 * Horrific Relevation reveals that
 * In MAR, the world hasn't been living the best of times because, while the Chess didn't lose the first fight, the good also didn't win, and the leader of the bad team escaped, so the world's been living in fear of the Chess rising again. Doesn't help that their captain (not the leader though) was a zombie and the world was told that he'd be back after a while, so they've been living in fear for the second War Games to start, which is where the story catches us.
 * Furthering this trope, is the fact that if the captain of either side loses, that side loses the entire game. The Chess have an entire army, so their captain stays out of the fighting until the sorting algorithm of evil forces him to join in. Team MAR only has one or two extra players, forcing the captain to participate in almost every round.
 * In Star Driver, Takuto could never lose a single battle (which occurred Once Per Episode) or the Glittering Crux would instantly succeed at their goal of kidnapping the Barrier Maiden and release the seals.

Comic Books

 * There's a version of this in Transformers Last Stand of the Wreckers: Shockwave tells Overlord that he isn't the first Phase Sixer (or One Man Army) to bait Megatron. Overlord notes that he only has to be the last — the one who wins. And while in this case both parties are evil, Overlord is probably the worse of the two.
 * The backstory of the original Mark Millar comic book Wanted.
 * In one Batman: Gotham Adventures comic, The Joker claims that he's always let Batman win, because if the Joker wins once, Batman dies and can no longer be played with, but every time Batman wins, the Joker is simply sent to Cardboard Prison and it's only a matter of time before the game begins again. Of course, this is The Joker talking.

Film

 * In The Fifth Element, the personification of evil springs up every 5000 years. The titular element has to be used in conjunction with the four element stones to wipe it out, but it keeps coming back. If evil gets ahold of the four stones, however, it can wipe out all life in the universe. No recovering from that.

Literature
""Yes," said the baby, opening its eyes. "The Doctor suceeded. This time.""
 * In the Discworld Vimes thinks this about the repeated assassination attempts he suffers. However, in the end the Assassin's Guild declines to accept more commissions on him. One reason is that his role in the management of the city has grown vital enough that they don't want the disruption removing him would cause. The other is that their repeated failures were just getting embarrassing, and assassins value their own lives highly.
 * The Last Hero Cohen the Barbarian says it doesn't matter if he fails to kill the Discworld Gods, it just matters he tries because "Next time someone'll try harder".
 * Roger Zelazny subverts this for A Night in The Lonesome October. The initial assumption is Pretty well justified, as the Evil in this case is the entire collection of Lovecraftian Eldritch Abominations. But it is revealed in an offhand way that Good has lost before, albeit not in recorded history, and the lead character has contingency plans in case Good loses again.
 * This seems to be the case in the world of The Wheel of Time series. Over aeons, The Pattern endlessly repeats itself, and the same heroes end up being reincarnated to fight the forces of the Dark One. When the forces of Light win, the Dark One goes back to being Sealed Evil in A Can. If the Dark One ever wins, the Pattern will be destroyed, ending the cycle permanently.
 * However, it gets really weird. The Dark One only needs to win once to break out of his prison across its myriad permutations, but as long as one reality that stands unbeaten the Dark One cannot break free. It's weird.
 * There are also hints within the story that he has won before, only to find his efforts rendered futile as the Reset Button was pressed.
 * This is also central to The Dragon's view of life. A former philosopher, he became Genre Savvy both to the endless repetitions of the Wheel and the fact that Evil Only Has to Win Once, and decided that not only was saving existence impossible (since the Dark One has an eternity to get it right), but since the cycle only repeated variations of the same events over and over again with no hope of true change, existence wasn't worth saving.
 * Roland of The Dark Tower is seeking the aforementioned Tower to save it.
 * The implication is that it cuts both ways.  Evil Only Has to Win Once... but GOOD also only has to win, really decisively WIN, once as well.
 * The Cthulhu Mythos is full of Eldritch Abominations lurking in other dimensions, many of whom could destroy the world (or the entire physical universe) if they felt like it. So if some insane cultists ever succeed in waking one of them up, it's all over red rover.
 * The Lord of the Rings averts the trope in the specific context of it's Big Bad, but follows it concerning evil in general. As Gandalf explains, if Sauron wins his victory will be so complete that none can foresee his fall while the world lasts, but if he falls, he will fall so low that none can foresee him rising ever again. However, As Long As There Is Evil (which there will always be, since it was woven into the fabric of creation itself by Morgoth), "the shadow will ever take another shape and grow again."
 * In the Doctor Who Past Doctor Adventures novel Festival of Death, the Doctor and his allies prevent an Eldritch Abomination from eating the universe. So far, just another day in the office for the Doctor. But there are several time loops involved, so there's a sense in which the adventure is happening over and over again forever -- and if the Doctor and his allies slip up even once, it's goodbye universe.
 * In the Doctor Who Past Doctor Adventures novel Festival of Death, the Doctor and his allies prevent an Eldritch Abomination from eating the universe. So far, just another day in the office for the Doctor. But there are several time loops involved, so there's a sense in which the adventure is happening over and over again forever -- and if the Doctor and his allies slip up even once, it's goodbye universe.


 * Averted in Villains by Necessity itself an inversion of the Standard Fantasy Setting. The forces of Good did win the ultimate battle two centuries ago and have been turning the world into a Sugar Bowl ever since. A druid convinces some of the few remaining evil people in the world to restore the Balance Between Good and Evil by releasing the Sealed Evil in A Can.

Live Action TV
"Buffy: Not enough.
 * In Buffy the Vampire Slayer, Spike asks Buffy how many vampires, demons etc she thinks she's killed.

Spike: (nods) And we just keep coming. But you can kill a hundred, a thousand, a thousand thousand and the enemies of Hell besides and all we need is for one of us- just one- sooner or later to have the thing we're all hoping for.

Buffy: And that would be what?

(Spike leans in close and whispers in her ear.)

Spike: One... good... day."


 * But of course that would only be an example from Buffy's personal point of view. In the big picture, "one Slayer dies, another is called."
 * The trope is in use when number of villain plans that would if successful destroy the world, unleash hell on Earth, etc.
 * Unfortunately...
 * They aren't more evil, just more permanent.

Tabletop Games

 * Doom: The Board Game by Fantasy Flight Games features a campaign mode where the Space Marine players have to win all five scenarios while the Invader player only needs to win one in order to win the whole campaign. The Marines get to keep their equipment between games, though.

Video Games

 * Non-plot variant in Unreal Tournament: in Assault mode's rules. The match consist of two rounds, one defense and one offense; if the attackers in the first round win, they swap roles with the defenders and the new attackers must win faster. Example: your team starts with offense. If you lose the offense round, you instantly lose the match. If you win the offense round but lose the defense one, you lose the match as well. Under the same rules, if you were to start with defense you could win the match either by winning defense OR losing defense but beating the other team's time in offense... but if you play against the AI, you always start with offense and therefore you can't afford to lose even once. This can be particularly frustrating since the bots on your team are severely handicapped in Assault mode.
 * The Soul Series. So far, the Soul Edge (the evil sword) has been shattered or destroyed at least three times (Soul Edge/Blade, Soul Calibur 1, Soul Calibur 2), twice by the Soul Calibur (the "good" sword). What happens each time? The sword just breaks into pieces (each one every bit as evil as the whole sword) and eventually reforms itself, stronger than before. Now, what happens in every ending where the Soul Edge wins instead? The world gets hosed, that's what. Even worse, in SCIV, we learn that the Calibur is actually just as evil, but with a penchant for Order rather than Chaos.
 * Mortal Kombat is mentioned in the trope description but it is actually a subversion. By the time the events of the original game play out (after being RetCanon'd by the movie), not only has evil already won once, but has been doing so for the past 500 years or so. The tournament that is being documented by the game is actually the tenth such tournament following a string of nine victories by current champion Goro. If the Earthrealm fighters fail to crown a champion of their own in this tournament, Shao Kahn and Outworld has free rein to invade and conquer Earthrealm. So in Mortal Kombat's case, it's "Evil Only Has To Win Ten Times...But This Is The Tenth Time."
 * Castlevania operates on this principle, though so far it hasn't had to actually employ the trope. Every hundred years, Dracula's castle reappears and whoever in the Belmont clan (except the one time that Morris guy had it at the start of the 20th century) possesses the Vampire Killer whip has to storm the castle and slap Dracula back to the abyss. Should they fail...well, everyone's going to pay the price but all Dracula has to do is wait another hundred years (a stone's throw in vampire time) and he gets to try all over again (until 1999 and a well-timed eclipse screwed him over for good).
 * Arguably inverted in the Mass Effect series, where
 * Inverted in Final Fantasy I, where it's revealed at the end that Evil has won over and over and over again for nobody knows how long. The only way for the cycle to end is for Good to win.
 * While Dark Souls runs on Grey and Grey Morality, this is invoked by the games central conflict. In order to keep the First Flame alive, the followers of Gwyn must always keep the cycle of sacrifice going in order to continue the Age of Fire. However, all the followers of Kaathe and the Pygmy have to do is merely wait for the First Flame to die.

Web Original

 * The sorcerer Dagon, from the Global Guardians PBEM Universe, pretty much had just one evil plan that he used every time he went up against the heroes: release the Great Old Ones from their extra-dimensional prison so they can rule the Earth once again as they did billions of years before those johnny-come-lately humans (who, by the way, will be served up en masse as hors' dourves at the "Happy Get Out of Extra-Dimensional Prison Day" party). He's been beaten every time so far... but he only has to win once.
 * The world is in for any variety of apocalypses if the SCP Foundation fails to contain some threats. e.g. SCP-231-7 only has to give birth once.

Western Animation

 * Man Of Action's Generator Rex joins the party with the introduction of the meta-nanites,, gets their hands on them it's game-over.
 * It turns out that
 * If either of My Little Pony Friendship Is Magic's Big Bad Mad Gods (Nightmare Moon for Season 1, Discord for Season 2) had achieved their respective goal it'd be lights out forever or, in the latter's case, Equestria would've turned into a World Gone Mad.

Real Life
"Today we were unlucky, but remember we only have to be lucky once. You will have to be lucky always."
 * The 1984 Brighton hotel bombing, which didn't kill the top British officials who had been targeted. When the Irish Republican Army claimed responsibility, they said:


 * In speeches about the 9/11 attacks and the subsequent War On Terror, George W Bush and various officials have said this about Al-Qaeda and other terrorist groups.
 * More generally, ideals like freedom and equality are very recent inventions, only rising in the last few hundred years. In fact, the civil rights for the blacks in the USA are only a half a century old. It's also true that technology gives increasing powers to track (and crush) dissidents to the police. Given this, it's possible that the modern world was an accident of history, could be squashed at any time, and once crushed can never rise again.


 * Dred Scott v. Sandford gave something like this: Scott was a slave, and got his freedom by a court case--he'd been taken into a state where slavery was illegal, and there were precedents of slaves being freed this way. However, the widow appealed to the Supreme Court, and won, making Scott her slave again. And then he couldn't get an appeal, because he was a slave--property, not person.
 * And...and also because the Supreme Court is a court of last resort. Not even a person would have been able to appeal from it without a Constitutional amendment (which is what eventually happened).
 * Wellllll, not quite. The widow, between the time she filed suit and the time things got to the Supreme Court, had remarried to an abolitionist member of Congress from Massachusetts, who found out that his wife owned the most famous slave in the world from the newspaper- and way too late to stop the suit from proceeding. After the decision came down, he would have freed him immediately, but he couldn't, because Scott came from Missouri and only a resident of Missouri could emancipate him. The Congressman shoved Scott on a Missouri family that had owned Scott many years prior, and they freed him a whopping two months after the Supreme Court ruling. The Congressman didn't even attempt re-election.
 * Computer security experts versus crackers. For crackers to get into a system and exploit or vandalise it, they only have to find one flaw in the software that can be exploited. The computer security experts, however, have to make their software flawless for it not to be exploited. Given the complexity of modern software, this is to all intents and purposes impossible. Even a hypothetically flawless system could be cracked wide open should a cracker discover the credentials of a user with high level access and a lackadaisical attitude towards passwords.
 * Or just use Rubber Hose Cryptography: beat the password-keeper until he gives you the password.
 * Environmental protection. Once damage is done and the natural state of a place is gone, its gone, and if recovery is at all possible, its usually too expensive and what comes back won't be what used to be there. The bill setting an area aside must be defended from illegal trespassers and legal alterations every day of the year for the rest of time for its backers to get their way, but bulldozers only need to go through once for their backers to win. Some things eventually recover given several hundred thousand to several million years, such as chemical contamination, but they're as good as permanent from a human residence perspective, and species loss is permanent.
 * You could argue that the concept of entropy is this trope, written into the fabric of the universe. It always takes some amount of effort to create or maintain order in a chaotic system, but all it takes is a little decay for those systems to fall back into chaos again. In other words, it's always easier to destroy something than to create it or keep it from being destroyed.
 * During World War II, the phrase "The bomber will always get through" was coined, as applied to air defense strategy. In other words, even if you have the best air defense available, some of the enemy planes will get through, and given the damage that one fully loaded bomber can do strategically, economically, and most of all psychologically, spending countless numbers of dollars to improve your air defense from a 50% kill rate to a 75% kill rate will have little effect bonus for your war effort, and cost a staggering amount. During the Cold War, this applied to nuclear deterrence, and why there were a fair amount of guffaws among nuclear experts about a so called high-tech "Missile Shield" (a/k/a Ronald Reagan's "Star Wars" defense). Given the amount of nuclear weapons available, even if your missile defense had a 99.999% success rate, an amazing feat in itself, you would still have a good 20 or 30 or maybe even more megaton nuclear weapons coming down on your populace if the other guy decided to launch. Later, this would be applied to current counter-terrorism efforts. If just one suicide bomber successfully gets through, then, Game Over, your counter-terrorism efforts failed. Even if you just captured 1,000 terrorists red-handed and sent them off the Guantanamo Bay. On that happy, note, only about 5% of cargo inbound to the United States is screened at all. Sweet dreams!
 * Do note, even if a terrorist wins once, it wont mean the defeat of his enemies. And with suicide bombers, it defeats at least one terrorist. Be positive.
 * This is the prime tenet of defensive driving. You can drive the same route every day to work and make the same turns but it's always worth stopping and checking every time you approach them as the one time you decide that it isn't worth your time could also be the time that sends you to the hospital in a neck brace.