To all of the people who denied the fact that The Lich started the war, look at this picture from the trailer...

S5 e1 The Lich with other shadow figures

The Lich emerging from a cloud shaped like a mushroom with souls coming out of it. Demons walking behind him.

And all of the smashed buildings behind them is most likely the after picture of the war. So here's the backstory: When The Lich went to the Wishmaker to wish for destruction of all life, that was the starting of the war. So, when Finn wished for The Lich to never even ever exist, the Lich doesn't exist, the war can't happen, and Finn and Jake stay in their normal boring lives. I think thats more than enough proof. Any questions? Please comment.