You know that as soon as we invent a time machine, the first thing we're going to use it for is to destroy our enemies retroactively. (Well, maybe the second thing.)
Time wars are a huge part of time travel stories. As soon as two opposing sides both have time machines, they're going to keep going back and over and over, to try and change each other's history. Here are the most horrifying — and hilarious — time wars in any continuum.
Star Trek: Enterprise
Does anybody actually understand what the heck the Temporal Cold War was about? Or who that shadowy guy from the future was? It hardly matters — this storyline culminated with lumpy-faced aliens in Nazi costumes, and that's more than enough to justify its existence. Apparently someone in the 29th Century was trying to prevent the Federation from ever being created, and the adventures of the first starship Enterprise — which probably had a totally uneventful run in its original timeline — were a focal point, as different sides kept trying to change events.
Doctor Who, "The Curse of the Fatal Death"
We may never know what the Time War was like, in which the Daleks and the Time Lords battled for control over the time-space continuum. But it probably looked a lot like "Curse of the Fatal Death," a special one-off comedy episode written by Steven Moffat in which the Doctor and the Master spend a lot of time going back and trumping each other by bribing an architect — in retrospect, this special encapsulates a lot of Moffat's approach to the series, especially his love of "timey wimey" storytelling.
Abe and Aaron have a working time machine, which goes back in time six hours — and they use it to get rich and influence people. But eventually, they start having disagreements about the proper use of time travel, and whether they should be using it at all. Soon they're both traveling back and trying to edit their own pasts. But impersonating your own past self turns out to be a difficult challenge.
Fall of Chronopolis by Barrington J. Bailey
The Chronotic Empire wants to extend its power and its religion to all of time and space — but the Empire runs into difficulty when it attempts to conquer the Hegemony. Soon the Chronotic Empire and the Hegemony are fighting a time war, each of them trying to rewrite the other's past. By the end of the book, it becomes clear this time war has played out over and over again, with different details each time, because people in the far future have locked the Chronotic Empire in an isolated time loop, as a warning to others of the dangers of meddling with time.
The Stainless Steel Rat Saves the World by Harry Harrison
The supervillain known as He has gone back in time to 1975 to try and win a Time War against the Special Corp, including "Slippery" Jim DiGriz. Jim has to travel back to stop He, and winds up chasing him to the Napoleonic wars in which Napoleon has conquered London. Eventually, Jim begins to believe that he may have brought about the creation of He through his own time traveling.
Kang the Conqueror
I can't even begin to summarize the adventures of this Fantastic Four and Avengers villain here, but he frequently travels around through time — and between universes — seeking to conquer everything. He became an Egyptian pharoah and actually managed to dominate the 40th Century, but heroes of Marvel's 616 universe are usually there to stop him and fix any damage to the timelines. Mostly.
During his solo series, which started after 52 in 2007, the fame-whore superhero from the future becomes a time-traveling secret savior, trying to fix the timelines wherever they go wrong with the help of Rip Hunter and his robot friend Skeets. Along the way, he fights a number of time-meddling villains, including Rex Hunter, who want to change the time stream for the "better."
"The Men Who Murdered Mohammed" by Alfred Bester
This isn't strictly speaking a story of time wars — rather, it's a bunch of time travelers who all went back and tried to alter history, only to find that the more you meddle, the less you matter.
WikiHistory by Desmond Warzel
This beloved oft-quoted online short story is just the notes of a message board whose members keep having to fix each other's time distortions. Why does everybody always want to go back and kill Hitler? READ BULLETIN 1147, PEOPLE!
In this "meta-time strategy game," players can actually travel back in time to anticipate their opponent's attacks, making the course of the game extremely confusing and tactically difficult. Says the game's website:
How would combat change if you could foresee the future? If you could prevent your past mistakes from ever happening? What if your enemy could too? Achron is the first and only game to feature single-player and multiplayer free-form time travel. It is the world's first meta-time strategy game, a real-time strategy game where players and units can jump to and play at different times simultaneously and independently.
The Lords of the Sands of Time by Issui Ogawa
Humanity is fighting a desperate war against some evil aliens in this Japanese novel — and just when it seems like victory is possible, the ETs invent time travel and go back in time to wipe us out before we even begin. Luckily, our own artificial intelligences figure out how to duplicate the alien time travel process. As our review of the novel explains:
This is a blue-hot Temporal War with not just continents or eras as battlefields, but entire timelines. Orville and his fellow Messengers must recruit whole civilizations as cannon fodder, racing to upgrade their technology and stripping whatever resources they can against the tireless onslaught of the ET machines. When things look doomed, the Messengers have to kiss an entire world good-bye and set off for another multiversal beachhead to start the whole process over. Through warped versions of pre-fuedal Japan, the American Civil War, and a Mid-20th Century that Harry Turtledove would be proud of, Orville keeps fighting the good fight.
The Big Time by Fritz Leiber
This is one of the most famous time-battling stories of them all — two factions are at war, using time travel, and they both recruit soldiers from every time and place on Earth. They fight across time and space, trying to dominate reality, and their poor human footsoldiers are just caught up in it. A similar theme appears in Poul Anderson's Corridors of Time.
Star Trek: Voyager, "The Year of Hell Parts 1 & 2".
This is somewhat more satisfying than the Enterprise storyline — except for the reset-button ending. (Trek also does this sort of thing quite a bit in other stories, including Star Trek: First Contact, with the Borg trying to change history.) This time around, the Krenim are using time technology to erase their opponents from history, creating time incursions that reshape the whole of their area of space — until Captain Janeway shows them what one good starship crash can do.
The Terminator and sequels
And then finally... there's arguably the most famous dueling-time-travelers story of them all — the A.I. Skynet wants to ensure its victory over the future humans, so it keeps sending killer robots back in time to make sure John Connor is never born, or never grows up. In the television series The Sarah Connor Chronicles, there are a whole lot of missions back in time, with various objectives that get ever more confusing.
Thanks to Tansyr Roberts, Rus McLaughlin, Patrick Scaffido, Kiah Pierson, Austin Grossman, Kostazu, Tom Coates, Tristan Salazar, Stephen Black, Julia Lunetta, Sunil Patel, Justin Cascio, Chris Hsiang, Allen Varney, Matt Wilson, Andrew Liptak, Michael Grella, Minal Hajratwala, Harry Connolly, M-D November, Stephen Blackmoore, Chris Watkins, Amanda C. Davis, and everybody else who suggested stuff for this list!