Alien Space Bats

""1865 - Alien bats from outer space bring the fruits of their technology to their brothers, because they have heard Elvis Presley on the radio, and think that the south should indeed be free. It ranks slightly higher than a '63 CSA victory. Indeed, I think I will call it "Bats of the South", and make it into a four book trilogy.""

- Alison Brooks, discussing the probability of various Alternate History Confederate victories in The American Civil War on soc.history.what-if

An Alternate History trope dealing with the divergence of a timeline. The phrase is widely used on Alternate History websites. If the point of divergence is an extraordinary or supernatural phenomenon, Alien Space Bats are responsible. If history changed due to historical happenstance it was just For Want of a Nail.

Alien Space Bats is in a sense the opposite of Deus Ex Machina: where Deus Ex Machina is the introduction of an implausible element outside of the context of the narrative to resolve a plot conflict, Alien Space Bats are an implausible element outside the context of the narrative introduced in order to set up the main plot conflict or setting of the story.

The phrase was originally coined by the late Alison Brooks as a sarcastic comment on ridiculous Alternate History timelines with no realistic chance of happening without some sort of Deus Ex Machina as implausibly contrived as bringing in a bunch of Sufficiently Advanced Alien bats. (The definitive example of a plan whose success would be hopelessly implausible without something like alien space bats is Operation Sealion.) It was only later that it came to mean "explicitly magical or science-fiction what-ifs."

The trope may also apply when the point of divergence isn't actually supernatural, but so wildly implausible that it might as well be that A Wizard Did It.

Note that Tropes Are Not Bad: this can and does lead to some excellent yarns.

A frequent mechanism by which Alien Space Bats intervene in human history is Mass Teleportation. When on a small scale, their intervention may leave people Trapped in the Past. See also Never Was This Universe.

Not to be confused with Goddamn Bats. Or an alien Batman.

Anime and Manga

 * Zipang diverges from history in the wake of the Battle of Midway when a modern Japanese Aegis destroyer is sent back to 1942 by a Negative Space Wedgie.
 * The world of Fullmetal Alchemist (or at least that of the first anime) diverged from the normal world when alchemy was discovered.

Comic Books

 * Watchmen: The exact point of divergence seems to be the presence of "costume heroes", which isn't too fantastic (none of them have any superpowers), but most of the really major differences can be attributed to Dr. Manhattan, whose appearance marks the point where the course of global politics and history dramatically shift.
 * Marvel 1602 has everything fairly normal up until the future Marvel Universe suddenly imposes itself on the past.
 * One alternative world The Authority fought diverged when blue-skinned aliens arrived in Italy during the Renaissance.

Fan Works

 * A Scotsman in Egypt, an epic Total War After Action Report, starts off with two drunk Scottish princes invading Egypt...and winning. And they (and their successors) don't stop there.

Film

 * The movie Lifeforce literally has these: The Ancient Astronauts were really energy vampires.
 * The Final Countdown and a few imitators also have some sort of Negative Space Wedgie toss an individual, a country, or a military force back or forward in time so they can change history.
 * The Final Countdown was a Stable Time Loop. Imitators, however, may not include this aspect.
 * Possibly referenced Justice League: Crisis on Two Earths, when Superwoman is trying to decide which alternate Earth to send Batman to, she glosses over one where humanity has "mutated into hideous winged creatures of the night".
 * X-Men: First Class reenacted the Cold War with mutants.

Literature

 * The 1632 series has a small modern American town physically relocated to 17th-century Germany by some process that the author discusses no further than to vaguely say that advanced physics could probably explain it. It does give a tiny bit of exposition about the Space Bats in question (an alien species that thinks of creating temporal anomalies as being True Art), and notes that eventually, they get their just desserts at our descendants' hands for the general hazard their art poses, but that's all on page 1 and they never appear again.
 * John Birmingham's Axis of Time trilogy, inspired by The Final Countdown (see above), depicts a military task force that gets sent back in time from 2021 to 1942 as a result of a failed experiment on one of the ships in the task force.
 * Without Warning and its sequel After America also by John Birmingham, set in 2003 and after, features a wave of unknown energy that causes the population of most of North America to be suddenly disintegrated. Other, non-primate animals are either unaffected or destroyed on a seemingly random basis.
 * In Harry Turtledove's The Guns of the South, the Confederate States win the American Civil War because time-traveling South Africans give them AK-47s.
 * Ravage uses an identical premise, but falls under the category of future rather than Alternate History, as it takes place in the 21th century and was written in 1943.
 * The Belisarius Series is a good example; its divergence is the result of warring AIs FROM THE FUTURE.
 * Let's be accurate: Its divergence is the result of warring AIs FROM THE FUTURE, one of which was sent to prevent any timeline close to the one it came from and the other of which was sent back to stop it.
 * The major point of divergence in the Wild Cards franchise is the outbreak of the eponymous virus on Earth, which bestows superpowers on its victims (that is, if you can avoid the horrible death part).
 * In an intentional Homage to this trope, Ken MacLeod's Learning the World is set on a planet inhabited by actual Alien Space Bats—to whom humans are the mysterious alien visitors who change the course of history.
 * Kim Newman's Anno Dracula series and Neil Gaiman's A Study in Emerald have respectively Dracula and HP Lovecraft's Old Ones as rulers of The British Empire instead of Queen Victoria. In Newman's, Dracula does this with Victoria's own approval, as her regent.
 * S.M. Stirling is noted for this trope, probably because he was a regular reader of the newsgroup where the term was coined:
 * The Draka has the initial divergence of American and French royalists being sent to the fictional colony of Drakia. It then has a number of others, such as the existence of an incredibly complete cache of classical literature in Western Africa, and the spontaneous appearance of several technological advances in a culture with little incentive to have them. For example, they send steam-powered warcars to help the Confederacy, and have enough dirigibles to launch an air raid that kills 50,000 people against Russia in the 1880s. They also have atomic bombs by 1944, but so does the United States.
 * Island in The Sea of Time starts with the Event: Alien Space Bats sending Nantucket (and a big ellipse of ocean surrounding the island) back in time to the Bronze Age.
 * The Emberverse novels: in the 1998 from which Nantucket was taken, the same Alien Space Bats cause all industrial-level technology to become useless. Lampshaded as some of the characters explicitly use the term "Alien Space Bats" as a label for whatever unknown force caused most human technology to suddenly stop working.
 * The Sword of the Lady, these particular Bats are revealed to be.
 * The Lords of Creation series is set in an alternate history where Mars and Venus are habitable (having been made so centuries ago by the eponymous advanced alien race, for reasons not yet revealed).
 * Harry Turtledove's World War/Colonisation books - the point of divergence is aliens invading during World War II.
 * In the Roger Zelazny book Roadmarks, the time-traveling main character keeps attempting this to fix Thermopylae (in the story the Greeks lost) but the Time Police keep catching him.
 * Elizabeth Bear's  New Amsterdam has alien space bats in the form of magic warcraft used by Native Americans, preventing Europeans from settling the Americas except spottily along the coasts.
 * Thor Meets Captain America by David Brin has Nazi Germany essentially winning World War II because they were able to summon the Norse gods to fight on their side. Alien Space Bats was used to make a point here: this was the most plausible scenario the author could think of that would have the Nazis winning.
 * Steven White's Saint Antony's Fire starts off with Ponce de Leon discovering the wreck of an interdimensional UFO, quickly followed by the resurrected aliens allowing the Spanish Armada to successfully invade England.

Live-Action TV

 * A few Sliders episodes fell into this, with worlds where physical laws permitted magic and wizardry and dragons, whereas other worlds were For Want of a Nail. Still other worlds the Sliders visited combined these aspects.

Tabletop Games

 * The GURPS "Infinite Worlds" campaign has two major opposed alternate-reality-jumping factions (Homeline, our world circa 2027 if paratemporal technology had been invented in 1994, and Centrum, a recovered post-apocalyptic One World Order of Straw Vulcans with similar tech) often act as Alien Space Bats in other timelines to further their own interests (which right now is mostly screwing up the rival faction). The players are probably going to work for one or the other.
 * Shadowrun, as of Fourth Edition, splits off in 1999, when the Supreme Court grants certain major corporations "extraterritorial" status following a vicious food riot that turns catastrophic. ("Extraterrorial" means that Mega Corp property is not subject to national law.) Things really hit ASB levels when children start being born as elves or dwarves...

Theme Parks

 * The roadside attraction "Professor Cline's Dinosaur Kingdom" features an alternate version of the American Civil War where the Union army tried to arm itself with dinosaurs. It didn't turn out so well.

Video Games

 * The Resistance series is based entirely around this trope, where the alien Chimeras arrive in the Tunguska event of 1908. In 1921, Russia initiated a communications blackout with the rest of the world, and built a wall against its European border called the "Red Curtain". In December 1949, the Chimeran forces invade mainland Europe. The first game starts with their invasion of England in 1951.
 * The backstory for Dawn Of Victory, a mod-in-development for Sins of a Solar Empire, is inspired by the Worldwar series in that it involves aliens invading during WW2 and proceeding to kick everybody's ass, except for a few isolated victories. Then nukes are developed and used, pushing the Scinfaxi to the Southern Hemisphere. History then proceeds similar to ours in the Western world, except there are three superpowers: USSR, Germany, and the Democratic Federation.
 * In Robo Aleste, the arrival of a mysterious foreign derelict ship introduces to Sengoku period Japan firearms, airships and Humongous Mecha.

Web Comics

 * The Ciem Webcomic Series takes place in the Gerosha multiverse (with four universes based on different interpretations of Candi Levens.) The four 'verses never interact, but their common Point of Departure from our history is the Battle for Gerosha. In later ones, the first Marlquaan storm serves as a secondary Point of Departure.
 * The First Marlquaan Storm, Zeran Portal Technology, and creation of Phexos and Meethexos through various means all count as Alien Space Bats.
 * The Hebbleskin Gang conquering Boonville and the National Guard building Gerosha on its remains after the battle, however, could be considered For Want of a Nail.

Web Original

 * Alison Brooks introduced the alien bats to soc.history.what-if in her Alternate History spoof Irony And Steal. Here, the bats descend on Manchester, England at the start of the Napoleonic Wars, having learned how to speak English from listening to future radio broadcasts, and supply tanks to Britain with which it can defeat France at El Alamein. Silliness ensues, including the Russian Czar marrying a bat, dirigible arms races, Lenin becoming a baseball player, and alien mutant ninja turtles replacing the population of Australia.
 * On the Spacebattles forums, an entire section of the site is dedicated to what they call "Random Omnipotent Beings," or "ROBs" doing precisely this, starting off many role-play threads.