The Mushroom War has happened in the the future, and here's how i layed it out:
The Mushroom War began with a world-threatening communist dictatorship from the future Soviet Russia. Similar to the reign of Adolf Hitler, Russia was rallied into an idea to take over the world. Bombing raids were scattered all over Eurasia. Many smaller countries were in confusion and inmatched decision against these unexplained attacks. Russians probably used high technology artillery alien to us, maybe like the UFOs from the Rainicorn-Dog Wars.
Later, when America joined the war, the tension was starting. Before it was called the Mushroom War, the war was probably called World War III or something. Though because the Russian "hitler" was restless, a primary war has turned into a nuclear war, World War becoming Mushroom War. Both Union Allied and Soviet countries are all trying to overpower and threaten each country will more and more nuclear artillery. Eventually the improper use of nuclear plants have tripled, and nuclearic chemicals have started mutating and polluting wildlife, and even mankind. Reckless the countries have become and bombs have fallen everywhere, devastating people, destroying cities, and mutating the world like never before. Large amounts of nuclear chemicals also damages climate change, and the world has become colder and colder in the north and hotter in the South. This explains the Ice Kingdom, which could be the leftovers of a European country (Ice King has a slight scandinavian accent). And the Candy Kingdom could be post-war Germany, since Princess Bubblegum is known to speak German. The Desert of Doom could be located in the South since it was once covered in ocean, due to the fact it's remains show old and broken down ships and whale bones, but nuclear climate change has soaked up the water like a sponge. The Humans are now endangered.
Well how do you explain the survival of wildlife and fableheim? Similar to the the path of Moses, The Cosmic Owl, sent down from Mars, probably led the animals to some magical oasis, like the the Rim of Heaven, to escape the ice age!( I'll explained the ice age later.) That may sound weird, but guess how they got Buddhist/Christian-like beliefs! Outside the "oasis" the Ominus Bomb has fallen blowing of the entire landscape of the future East Pacific, and almost all humankind have died or mutated. Rainicorns ate the remains of dead humans.
The Rainicorn-Dog Wars
The Dogs though, lost companions of the humans, needed a home. the Crystal Dimension their best option. The Rainicorns of the Crystal dimension, however, did not want to share territory with the Dogs, and sought war against them. The Dogs, dependent on survival, had no choice but to fight back, armed with leftover technology from their nearly-exctinct human friends. This started the Rainicorn-Dog Wars. (The Dogs most likely won the war, due to the fact that dogs survived, but i do not own Adventure Time, so tell me if anything has changed.)
The Big Freeze
So tell me, how did Finn survive in 1000 years? Well it's a long theory. If Finn is 100% human then he must have an abandoned family.
After bombs had fallen and the war ended, the biz has not ended yet. While most creatures are living in the oasis, the nuclear war has inflicted a vast, diverse, meteoritrical, weather-craving, climate-change. To make things short, the next ice age. Because such cold weather, and technology has been destroyed, the remaining humans have been relying on Neolithic life, or a life of a Cave-Man/Eskimo tribe. The distinction of this is easy to explain. Finn was a member of a later abandoned eskimo family living in this ice age. If you've seen the movie, Ice Age 2, you'll know what i mean. At last, the Earth is warming up, and the animals are ready to leave to oasis to the brand new world. I bet once they left the oasis, they went "Oooh..." or "Aaah..." and that where the Land of Ooo got its name. At the same time, Finn would be a few months old.
The Abandoning of Finn the Human
So tell me again, how did Finn survive in 1000 years? Well, Finn was abandoned as a baby as according to the Adventure Time episode, "Memories of Boom Boom Mountain". How? Thats the question I'll explain my theory on. Finn's Eskimo family could have been disbanded. There are billions of explanations, though thats not so important. (My first guess was that the family were pursued by a massive white bear, in which explains how Finn skinned a bear and got his hat, but i ditched that thought after i read another article on the Mushroom War.) Finn was probably abandoned on a tree in BoomBoom Valley, then when poopoo on the leaf he was laid on and fell down. When a family of friendly dogs took him in and raised him. Finn had met Jake and the rest of his adoptive family. Thus Finn had always been dedicated to nature, losing teeth by biting rocks and trees in curiosity, which explains his missing teeth. He also learned to swim and he became so energetic he hardly ever stops, shown in the episode "Power Animal". Finn (and his brother Jake) were also dedicated to their father, who was a great a hero, shown in the episode "Dad's Dungeon".
The Life of Finn the Human and Jake the Dog
After Finn and Jake's parents died, ( I made a guess that they died in the the Rainicorn-Dog Wars, but i have no proof.) Jake took Finn to live out themselves.( It is unknown what happened to Jake's brother, Jermaine.) Finn and Jake probably discovered the Tree Forts that once belonged to Marceline. Then they started and made a living out of slaying evil, going on righteous quests, and adventuring to distant lands. That is when the show starts.