War Fic

A genre of Fan Fiction in which we find our plucky young heroes fighting in large military conflict, whether they want to or not. These fics almost always involve the main characters joining the military in some way, and almost always involve a focus on the weapons, tactics, and politics involved in war and how the characters react to them—as opposed to the characters themselves. Of course, this is helped a long quite a bit if said characters are already in the military.

This comes up a lot in series involving immortals (where this often expands on said immortal character's Backstory) and/or Time Travel combined with Historical Fics.

Gun Porn is entirely optional.

Examples of War Fic include:

Anime and Manga

Downfall of Bleach fandom, takes a far more realistic outlook on the Winter War -- and is bitterly violent, in both ideaology and combat.
Winter War is another Bleach War Fic, this one focusing on La Résistance after Aizen's forces win the Fake Karakura Town battle, Seireitei is occupied, and the rescue party to Hueco Mundo fails.
Ranma Goes to War and its first (but not second) sequel.
Given what subject manner the canon content was original inspired by, it probably isn't too surprising that Axis Powers Hetalia has many fics of this nature, running the gambit of world history.
The Poke Wars series is, as the name indicates, a set of war stories set in the Pokémon universe. What is interesting to note is that it's not about war between humans, but between a faction of Pokémon that decided that humans are Complete Monsters that have to be eradicated no matter what, and the humans and Pokémon allied to them. The initial stories are more of a survival action genre, while the Expanded Universe details a proper war effort where both factions are truly established.
Brothers In Arms. The School Rumble gang goes to Korea! And it has a sequel.
Zero 2 A Revision is somewhat a Digimon 02 war fic, there things goes much more bloodier than the actual series. It gives us questions of what would happen if the Digital Emperor was more sociopathic and had nuclear weapons, if Demon attacked the Real World with a real invasion army, if Devimon returned from the dead and digivolved to a mega, if the Flood and the Covenant from the Halo universe decided to pay a visit, if Malo Myotismon had the power of a God and so on.
The TSAB - Acturus War: The TSAB goes to war against the Democratic Republic of Acturus.
The Faceless is a Death Note War Fic where the human race is Dying Like Animals while they kill each other over whether or not they should kill each other to serve the Gods of Death.
Considering Mobile Suit Gundam's (and its numerous series) war drama settings, as well as the War Is Hell message, its no surprise to see many fanfics set during wartime, whether they're side stories, sequels, or even prequels to the wars already seen.

Crossover

Reunions Are a Bitch is probably one of the best examples in the Intercontinuity Crossover category. Featuring what has got to be the most detailed (and, as a consequence, one of the longest) description and execution of a crossover between Stargate SG-1 and Battlestar Galactica ever made. For lovers of realism, politics, and massive space battles it's a must. Can be read here.
Trek Wars: The Furry Conflict is exactly that: a pitched conflict of good and evil across two galaxies in a dystopic alternate timeline where the UFP and the Empire are wailing on each-other with everything they got and everyone (save for a few Star Wars species) is a furry. Notable for being one of the longest running (and by longest, we mean it updates maybe twice a year) crossover series on the net.
PonyCraft: Friends of Equestria is a crossover of StarCraft II and My Little Pony: Friendship Is Magic (as StarCraft is a war game, well…). Here, Twilight also has to learn how to be a leader.
- There is also Ace Combat: The Equestrian War, with Rainbow Dash being The Heroine.
The Last of the Cliffhangers Unit featured the cast of Higher Ground in the Future Badass arc of a largely uncompleted Mega Crossover in which Scott's stepmother Elaine is a demon general and the Cliffhangers are a Special Forces Unit. The fic really only covers the beginning and the end and doesn't really going into the complexities of the actual meat of the actual war part of it. The second chapter of Curse of Life, Miracle of Death takes place in the same timeframe (the first and third chapters were largely self-insertion).

Films - Animated

Twilight Valley. Yes, a Land Before Time war fic!
- Out of the Shadows is even more of a war fic taking place in the Land Before Time universe there almost everyone dies, included the main characters.

Literature

There's a Good Omens fanfic series that sends Crowley to the trenches^{[dead link]}.

Live Action TV

Firefly fanfic does this quite a bit, with the Miranda wave being the catalyst for a second major war between the Alliance and the Independents.
The Gateworld Virtual Fleet: Exactly What It Says on the Tin, focusing mostly on the galactic and intergalactic conflicts in the future of the Stargate Verse.
In Doctor Who, the Last Great Time War has never been reviewed in canon, save as a Noodle Incident (for cost measures, the Time War lies according to Russell T. Davies above the budget of the whole BBC in a year). Needless to say, this means the Eighth Doctor has some fanfiction about this occasion.

Video Games

CombatEvolution is a Halo fanfic that attempts to rewrite a well known event in the Halo canon . . . with really, really, REALLY expensive weapons systems. If you like Gun Porn, you will love this fic.
- Speaking of Halo, it has a crossover with Warhammer 40k called Chains Of The Kindred.
Exoria is a distant, modern-day sequel to The Legend of Zelda series. Despite managing to stay true to the usual Zelda formula, such as item acquisitions, dungeon grinds, and boss fights, the author never fails to remind you that the story is set in the midst of a continental war, and things can get brutal.
Played with in the Zelda fanfic Liberty Rock. Link goes off to help hold a fort in a neighboring country's civil war, but rather than focusing on the tactics, politics, or weaponry, the main focus of the story is on the characters' days besieged in the fort dealing with monotony, stress, and dwindling hope.
Metal Gear Solid 3: Snake Eater fanfic The Joy of Battle is set during World War II, which fits with MGS canon. Unlike the majority of MGS fics, especially those taking place during World War II, The Joy of Battle averts most War Fic tropes in that the story is about the characters with war as a backdrop and a complication.
Tiberium Wars is one of the better known examples. A full-blown novelization of one of the most popular RTS franchises out there.
The Armored Core fanfic Armored Core: From the Ashes is set very closely after the events of Armored Core for Answer, and with the reformed ORCA winning.

Western Animation

A lesser known example, but still worth mentioning, is the Kim Possible fanfic Horrors Of War. Featuring everyone's favorite teen heroes, and Josh, traveling back in time to the Vietnam War. Obviously, a certain combat trope is in full effect here.