The Mushroom War

The Mushroom War (usually referred to as the Great Mushroom War) was an apocalyptic event that occurred roughly a thousand years before Adventure Time. The war crippled and eventually resulted in the near-annihilation of the human species and left their civilization in ruins throughout the Land of Ooo.

Origins
Before the war, human civilization was prevalent throughout much of what is known now as Ooo. Although the cause of the war remains unknown, its name suggests that nuclear weapons were heavily used during the conflict, and thus was most likely World War III. There is support for this theory in "Holly Jolly Secrets Part II," where one of Simon Petrikov's video diaries shows a plane flying outside his window. Although the scenery outside his window indicates no sign of devastation, it is implied that the war had begun and the aircraft was actually a bomber. The beginning of the opening theme also suggests this as it briefly shows the remains of nuclear bombs and other weapons in Ooo. In addition, most episodes show the remains of various armaments and pre-war technology throughout Ooo.

The war likely ended when the mushroom bomb was dropped along with the other nuclear weapons. The episodes "Finn the Human" and "Jake the Dog" support this, as Finn wishes the Lich, a product of the mushroom bomb, never existed. When the wish is granted, Finn is transported to a world where Simon has sacrificed himself to stop the mushroom bomb and freezes the entire Earth in the process. Despite some destruction, without the detonation of the mushroom bomb, humanity continues to exist. Also, in "Finn the Human," Farmworld Marceline suggests that the mushroom bomb likely concluded the war as she states that the world would have been annihilated if Simon had not stopped the bomb with his ice powers.

The war likely ended approximately one thousand years before the events of Adventure Time. Marceline's flashback in "Simon & Marcy" is explicitly stated in a time card in the episode to have taken place 996 years ago prior to the present setting. The exact year(s) that the war took place have never been stated, though it can be concluded that the Mushroom Bomb was dropped sometime after, at the earliest, 1985, as in "Everything Stays", Two Bread Tom, a surviving human, is depicted singing the theme song to the sitcom Mr. Belvedere, which premiered that year; in "Simon & Marcy" the Ice King is depicted singing the theme song and making reference to the sitcom Cheers, which premiered in 1982, further supporting the 1980s timeframe of the war. However, due to the appearance of a Generation 3 iMac computer in the episode The Lich, (which started becoming available to mass public consumption in 1998), it can be assumed that the war occurred sometime after the year 1998. This would also help explain why Internet-derived expressions and phrases are frequently used by characters. Modern smartphones are seen in Patience St. Pim's flashback of the human era, in "Elemental"; this could indicate that the Great Mushroom War began sometime after the 2010's.

Aftermath
The end of the war left much of the world devastated and devoid of much life. The infrastructure that had existed throughout the land had been mostly destroyed. The little human infrastructure that survived, such as girder bridges and office buildings, now lay in ruin. Evidence of the war appear in the form of abandoned military vehicles, ruined army blockades, bombed buildings and crashed warplanes. A great deal of human technology also remained scattered throughout the land. Abandoned and destroyed vehicles were a common sight in the devastated landscape and their rusting skeletal remains can be seen in the backgrounds of several episodes. Despite the great destruction, some humans managed to survive the war, such as Simon Petrikov (Ice King), Moe, the tribe of humans led by Two Bread Tom, and the family lines that eventually led to humans in the current setting of the series, including Finn and Susan Strong; Marceline, a half-human/half-demon, also remained as one of the war's only survivors. Nonetheless, most presumably perished during the war. In "I Remember You," Simon says to Marceline that they may be alone after the apocalypse, and "Simon & Marcy" depicts a post-war society devoid of any other humans.

Some individuals survived the war through special or supernatural means. Simon Petrikov was kept alive by the magic of his crown, as he states in his note to Marceline in "I Remember You". Hunson Abadeer, Marceline's dad, was seen with Marceline in a flashback in "Memory of a Memory" in the remains of a ruined diner, though considering that he is nowhere to be seen in Marceline's flashbacks in "I Remember You" and "Simon & Marcy" it is likely that he may have been in the Nightosphere during the war and its immediate aftermath. Therefore, it can be inferred that they were separated sometime after the Mushroom War. In "Everything Stays", it is stated that some years following the initial aftermath of the war, a major climate change developed, implied to have been a result of the war and the dropping of the bomb. Having detected abnormal atmospheric readings, Two Bread Tom's tribe of humans sailed away from their makeshift forest settlement in a renovated freighter in search of a safer place to thrive.

Despite the destruction from the war, the land slowly began to recover and soon life was once again allowed to freely inhabit the earth. Only a few signs of the nuclear devastation remained in Ooo. One example was in the episode "The Vault," where Shoko helps Princess Bubblegum establish her new kingdom by covering up an irradiated river at the site of the Candy Castle. In the process of covering it up, Shoko becomes disfigured and mutated by the effects of the waste and appears as she had in premonitions in earlier episodes of the series. The episode implies that the Candy Kingdom is built atop a pond of nuclear waste.

Along with the rise of new civilizations, such as the Candy Kingdom, a diverse variety of intelligent life settled down in Ooo. Many were able to recover or develop technology for their own use. Examples of this are universal translator devices 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 & Marcy" which premiered that year) or are more recent inventions. Despite this, most human inventions, especially those related to transportation, remained lost to the inhabitants of Ooo. In "The New Frontier," Jake mentions this when he says that "rocket ships haven't been reinvented yet," implying that the technology to build conventional space-faring vessels was lost during 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.