|The Mushroom War|
|The Earth after the war.|
|Name||The Mushroom War|
|Introduced In||"Slumber Party Panic"|
|Latest Appearance||"The Great Mushroom War"|
The Mushroom War (Also Called: The Great Mushroom War) is the cause of the post-apocalyptic condition 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 the mushroom shaped cloud produced by nuclear weapons. 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.
The Mushroom War was most likely started by humans warring against each other on a nuclear scale (such as during the Cold War or the end of WWII). In "Holly Jolly Secrets Part II," one of Simon's video diaries shows a plane flying outside his window. Although the scenery outside his window indicates no sign of devastation, it can be inferred that the war had just started, and the plane is in fact a war plane. It can also be inferred that the Mushroom Bomb brought about the end of the war, creating the Lich. 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). Marceline, Abraham Lincoln, Moe (now a cyborg) and the Ice King (Simon Petrikov) are the only known characters to have survived the Mushroom War. Hunson Abadeer, Marceline's dad, was seen in a flashback in "Memory of a Memory," showing him and Marceline after the War but before the establishment of the Land of Ooo. This shows that he is, technically, a survivor of the War as well, but he may have been in the Nightosphere when the Mushroom Bomb was dropped. This can be further understood in "I Remember You," when Simon says that they may be alone after the apocalypse. The last piece of evidence to this is that in the flashback during that episode, Hunson was nowhere to be seen and Marceline is seen crying while mouthing "Daddy." Therefore, it can be inferred that he abandoned or lost her sometime after the Mushroom War.
In the "Finn the Human" episode, Farmworld Marceline states the Mushroom Bomb was dropped 1000 years ago. In the Farmworld timeline, Simon Petrikov froze the Bomb in midair, though it came so close to contact that it crushed him. Current Marceline is also 1000 years old. This means that in the normal Universe of Adventure Time the Bomb would have also been dropped around 1000 years prior to current events.
In "The New Frontier ", Jake mentions that "rocket-ships haven't been reinvented yet", implying that the technology to build conventional space-faring vessels was lost in a past event, most probably the Mushroom War.
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 prewar technology (the year of the war was never stated, but we know that it must have happened sometime after 1982 because the Ice King is heard singing the theme song and making reference to the sitcom "Cheers" in the episode "Simon and Marcy" which premiered that year) or are more recent inventions. All throughout the Adventure Time series there has been much evidence of human civilization.
In "I Remember You", the Ice King is shown to have a drawing of earth with a giant meteor headed towards it, possibly resulting in the missing chunk- although this may be referencing the asteroid in the episode "Wizard."