Mushroom war was bad, but hey. No one was left who could remember how it had happened, at least, no-one who would do anything.
Or so Dr. Wily believed.
Marceline, some business men in ice. Maybe a few others outside the afterlife and the nightosphere. But it's been a long time since, long enough for baseline humanity to nearly go extinct, wars to be fought, and a lot of other stuff went down.
The present is pretty sweet. There's a cure for death.