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 is a survivor of the war due to her 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's 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, as well as a decaying arm 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 baby shoes and the Businessmen. Finn also uses a flamethrower built from two fuel tanks connected to a rifle by a tube.
 * 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.
 * 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 a 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 corner.
 * Storytelling: There are many remains of radios and debris scattered throughout a forest.
 * 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.
 * Death in Bloom: Debris of civilization, including numerous police vehicles, in the Land of the Dead.
 * Susan Strong: The Hyoomen Tribe live in the ruins of an underground city. 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.
 * Go With Me: The Drive-In Theater houses the remains of many cars.
 * Belly of the Beast: The remains of a 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.
 * Mortal Folly: The base of The Lich's power is an abandoned, 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. 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: In front of Marceline's Cave there is a torn up highway that acts like an awning. There is 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.
 * 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.
 * Hitman: Outside the Breakfast Kingdom is a desert with an abandoned train in 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 an abandoned 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.
 * Thank You: Outside the home of the Snow Golem appears to be a 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 he 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. There is 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.
 * Holly Jolly Secrets Part II: The last tapes played showed the Ice King as a human. 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. The Scandinavia region of Europe is mentioned by name. 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.

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 "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 could have happened sometime in the future considering the advances in technology.

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) and throughout the video, it shows him looking more and more like Ice King. It is known that the Ice King is at least 500 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. 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, but it is also possible that there could have been pockets of humanity that survived a wave of the war and attempted to rebuild, yet changed over time as the world mutated into the modern Ooo. Likewise, Marceline could also be lying about her age and is closer in age to the Ice King, but neither can be determined by way of merely the video. However, another solution to the timeline inconsistency could be that vampires remain in their preteen state much longer than humans do, and that Marceline could actually be much older than a regular human child when Finn and Jake encounter her in her memories.