Here. ;)
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.
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.