Mushroom War

The Great Mushroom War is the cause of the post-apocalyptic setting of the Land of Ooo. Not much is known about the war at the moment, but the inclusion of "Mushroom" in the title is probably referring to a mushroom cloud generated from a nuclear bomb. If so, then it is likely that the Land of Ooo and the creatures who inhabit it were created from the nuclear fallout that irradiated the land. This is further supported by the ruined pieces of modern technology scattered across the land, and the beginning of the opening theme (which briefly shows the remains of nuclear bombs and other weapons). It was confirmed by Pendleton Ward that Marceline and Ice King are survivors of the war due to their supernatural powers.

The first mention of the war in continuity was in "Video Makers," when Finn and Jake discover government copyright warnings before the movies they watch for their film club. Even though the civilizations that created these warnings would have been long gone by their time (and that films bought for home video usage generally are protected to be used for private usage only, which being held in a home would be considered as such), they decide to abide by them to avoid breaking the law.

Strangely, the Land of Ooo also contains technological devices that are far more advanced than what exists today, such as Universal Translators and Holographic Newspapers. It is unknown whether these are based on pre-war technology or are more recent inventions.

Evidence or clues of former civilization
In many episodes of Adventure Time, there are many hints or clues regarding the world and the civilization that existed prior to the Mushroom War, including as follows:
 * Adventure Time Theme Song: An opening shot showing various weapons and technological debris, including grayed land and undetonated nuclear missiles lying around (like a russian SS-13 ICBM on the left), and an arm reaching upward (most likely a zombie) in a tree trunk.
 * Tree Trunks: Traffic signs, which have since somehow mutated into Sign Zombies. It is unknown how this has happened.
 * Business Time: Ancient artifacts frozen in icebergs, including computers, a bike, baby shoes and the Businessmen. Finn also uses a flamethrower built from two fuel tanks connected to a rifle by a tube. When the Businessmen, along with Finn and Jake, are shot up into the sky, they fall back down revealing the entire Land of Ooo. Ooo seems to be a large island, with open ocean surrounding it.
 * My Two Favorite People When Finn and Lady Raincorn confront Jake and Tiffany, the bottoms of upturned cars can be seen in the ground.
 * The Witch's Garden: The remains of a highway system and a plane's wing can be seen near Gary's nest; the River of Junk is essentially a river of mundane pre-war artifacts. Near the River of Junk, a car door can also be spotted close by.
 * Ocean of Fear: Underwater city ruins and vehicles appear, and a brief scene shows a damaged aircraft carrier and a stranded tank- an even briefer scene depicts two dead bodies on a couch in the remains of a pre-war living room. Finn and Jake also take to the depths in a submarine. You can also see the remaining tops of cars while theyOcean_of_fear,_dead_people.png are floating over what might have been an ancient road.
 * City of Thieves (episode): When Finn and Jake are in Penny's tent, a double barrel shotgun can be seen sitting among a pile of gold coins. In addition, the stolen chest of coin from the tower contains a cell phone in the bottom left corner.
 * Storytelling: There are many remains of radios and debris scattered throughout a forest. The young bear is also spraying graffiti on what looks like an overgrown van.
 * The Other Tarts: Various boats and whale skeletons can be seen in the Desert of Doom, suggesting that it may have previously been a body of water.
 * The Silent King: The storyboard for "The Silent King " contains the first time The Mushroom War is mentioned by name in the series, heard when Gummy is reading from the rule book. However, this line was cut from the actual episode.
 * The Real You: A shot of Earth appears, and a huge chunk of it is missing. The missing piece of Earth could have been a result of the Great Mushroom War.
 * Death in Bloom: Debris of civilization, including numerous police vehicles, in the Land of the Dead. There are also crashed UFOs, possible hinting technology when the apocalypse happened was very advanced, or aliens. (UFOs were seen in Her Parents as part of the Rainicorn-Dog Wars but it is unknown if it is from that) They also take an escalator down to the Entrance to the Underworld, and a TV is in the ground when they "sneak" past the entrance.
 * Susan Strong: The Hyoomen Tribe live in the ruins of an underground city. In the underground city, a car, a refridgerator, and some sort of submarines/huge ships can be noticed. In the storyboard, a warning reading "Biohazard Danger" appears on the hatch, although this was cut from the episode. The inhabitants were possibly humans driven there during the war and later mutated. Also, in one brief scene, the camera pans over a pile of junk. Upon closer inspection, one can see what apears to be a robot/mech arm sticking out of the debris, suggesting that perhaps robots/mech were used during the Great Mushroom War. There is also what seems to be the front of a tank sticking out of the garbage when Finn and Jake first jump down the hatch. Looking even closer, you can see artificial UV lights strapped onto the pillars, suggesting that the underground city may be, in fact, an underground "bunker city" of sorts where the humans would survive while the war waged on above their heads.
 * Go With Me: The Drive-In Theater houses the remains of many aged cars.
 * Belly of the Beast: The remains of a damaged car rest in the throat of the monster as Finn and Jake return to confront the bears in the stomach.
 * Video Makers: Video cassettes feature movies such as Heat Signature that were made prior to the war. Jake even mentions the Mushroom War in the episode. Similar video cassettes and VHS tapes appear again as tapes of the Ice King's diary in the episode, Holly Jolly Secrets.
 * Mortal Folly: The base of The Lich's power is a disrepaired, ruined subway station with deserted trains and littered with human skeletons scattered on the floor as if they had been killed that very moment around the station. The Lich gets it powers from "Toxic Waste". Character designer Andy Ristaino has stated that these skeletons "were supposed to feel like they were east coast commuters ... all bundled up for winter." Outside the station is what appears to be a nuclear missile. There are also ruins of buildings in the background.
 * Mortal Recoil: While it is unknown where they would get them, Jake implies that the Lich's "Well of Power", which he recreated in Princess Bubblegum's bathtub with various chemicals collected by Finn, is a vat of toxic waste left behind in his former lair.
 * Heat Signature: The title card shows Finn and Jake standing on a tearing, fading billboard (Finn eating a sandwich and Jake spray-painting a picture of Lady Rainicorn) which stands in a destroyed urban background. In front of Marceline's Cave there is a defunct highway that acts like an awning. There are also the remains of an old car across from the cave entrance. Later, the ghosts take Finn and Jake to the top of what looks like a torn up skyscraper with a helicopter crashed on the roof. The skyscraper does have a helicopter "H" pad on top, and other tall buildings can be seen nearby.
 * Memory of a Memory: Many of Marceline's childhood memories appear to be around the ruined remains of civilization: she is seen as a little girl living in a ruined city with a derelict tank near her home, then later catches her father eating her fries at a destroyed diner. This could be the fallout of the Mushroom War, which could cause a continuity problem, or it could be wreckage from a previous military conflict that eventually escalated into the Mushroom War. In Marceline's memory of a little girl, the unknown city in the background can be seen on fire. Around Ash's house, there are many tires and other debris strewn about the area. There is also what appears to be a mobile home on the roof of his house, which has an aged tow truck balanced on top. Also, in her memory near the beginning, the trees still looked like those of present day as compared to the discoloration of the trees in other episodes which could have been the results of post apocalyptic radiation.
 * Hitman: Outside the Breakfast Kingdom is a desert with an abandoned railroad car near it, also a sewer pipe leaks out water for the moat around the castle.
 * The Monster: When Finn and Jake find the villagers, various metal tools are on the ground.
 * Wizard Battle: The arena for the battle seems to be a broken down football stadium.
 * Legends of Ooo: a game on cartoonnetwork.com, mentions the Mushroom War. If you click on the radio in Marceline's house, Jake states that, "They stopped making these before the great Mushroom War."
 * The Creeps: A portrait of what appears to be of New York City's silhouette.
 * Beautopia: Beautopia appears to be the ruins of a luxury resort, as seen in the title card. When Susan, Finn, and Jake arrive at Beautopia, it seems as if it was some sort of mall or luxury resort due to signs left on the sides of buildings such as "Eat", "Sale", and "Buy".
 * Thank You: Outside the home of the Snow Golem appears to be a snow covered bulldozer, which could possibly be an artifact from before the war. The house in which the Snow Golem lives could also be a remnant from before the war, as suggested by Tom Herpich's promo art for the episode, which depicts a family of humans (or humanoids) living in the house in a time before it was covered with snow.
 * The New Frontier : When Finn and Jake see a rocket built or found by Banana Man, Jake states that rockets haven't been re-invented yet when Finn is worried he might die. This (and the acedemic worms talking about Nietzsche's Ubermensch) indicates that not all knowledge premushroom war is lost. There is also a limousine buried in the ground near the home of the Banana Man.
 * Holly Jolly Secrets Part One: The place where Ice King buried his suitcase full of tapes looks like an old junkyard. Next to Finn was a boat that had a harpoon on the front of it. It was probably used for whaling. A truck, a car, and a small plane also lie near the Ice King and Finn. When Finn and Jake are digging for the suitcase, a shoe is hanging from a tree branch and a TV is seen underground where the suit case was buried. A trumpet is also seen tied to a tree branch above the Ice King.
 * Holly Jolly Secrets Part II: The last tapes played showed the Ice King as a human (Simon Petrikov).The Ice King's crown apparently turned him into what he is now, as seen in the videos. Outside the window that Simon (the Ice King) is talking by, a city is seen, undamaged. Simon said he purchased the magic crown from a dock worker in Northern Scandinavia, a region of Europe. During his video, Simon states "the power of the crown will save him with its frost." He mentions that he does not know what this means, suggesting that the video might have been recorded right before the beginning of the war. (*A sudden snowstorm is seen to start up outside the window shown in the tapes that quickly blankets the city. This could possibly be an allusion to the onset of a nuclear winter.) This may mean that the Ice Kingdom is the city. Simon also mentions that his former fiance, Betty, left him due to his irrational behavior caused by the crown. It is also possible to see a map of the eastern part of the Americas behind Simon in the first part of the video.
 * Marceline's Closet: When Finn and Jake are playing Hide and Seek, ("Cloud Hunt ") Finn sings a song that goes "Over the mountain the ominous cloud, coming to cover the land in a shroud, Hide in a busshel, a basement, or cave, but when the Cloud comes a' hunting, no one is safe!", maybe referring to mushroom clouds created by nuclear bombs in the Great Mushroom War. While they are hiding in Marceline's closet, the Spider that lives in Marceline's wall looks like it has many eyes, big fangs, and somewhat radioactive.

Timeline
It is unknown just when the Mushroom War took place. In the episode "Her Parents" it is stated that the Rainicorn-Dog Wars took place over "thousands of years." However, this does not prove when the Great Mushroom War happened, since the Mushroom War went on in the land that is now Ooo, but the Rainicorn-Dog Wars took place in the Crystal Dimension. Or alternatively Jake may have exaggerated for drama's sake and simply meant "a very long time" when he said that. In the same episode, Lady Rainicorn's father, Bob Rainicorn, says that he thought humans were extinct.

In "Memory of a Memory," Marceline appears as a young girl in a post-apocalyptic environment. In this scene, a burning city in the background suggests the Mushroom War occurred fairly recently, although plants growing in the wreckage of a house could suggest some time has passed since the calamity happened, but it has also been noted in nuclear testing and usage (e.g., Hiroshima) that the radiation encourages plant growth, and days after nuclear bombing occurs plants and flowers start to grow like weeds. Because Marceline has been confirmed to be approximately 1000 years old, this could mean that the Mushroom War took place just under 1000 years before the events of the series. It has been said that the present in Ooo takes place one thousand years in the future.

It could have happened sometime further in the future than the "present day" that the series is airing, i.e. much later than the early 21st century, considering the relics of more advanced technnology they sometimes find.

In Holly Jolly Secrets Part Two, Beemo puts on the last tape which shows Ice King as a young human, who tells the video diary about his fiancé Betty and a crown he wore that tells him secrets (Ice King's Crown). Throughout the video, he is shown looking more and more like Ice King. It is known that the Ice King is at least 600 years old in age, but the early part of his video diary as a human show him living in what appears to be human civilization, either before or during the war. It is seen through the window that the city is suddenly covered in ice, possibly an the onset of a nuclear winter, This could possibly be the beginning of the Ice Kingdom. It could be determined that Ice King is at least as old as Marceline if both survived the war. The discrepancy in the 400 years differance between the two has yet to address by the show's staff, however it could mean any number of things from a retroactive change by them, simply taking the 600+ to mean anything over 600 years including approximatily 1000 years, or even Marceline not being truthful about her age.