Cataclysm

Somewhere in the late 21st century, there was a series of cataclysmic events that wiped out a large portion of the population. Among the events included a nuclear apocalypse, but scholars talk about tsunamis, mega volcanoes, plagues, mass hysteria, EMPs, and many other things that went wrong at this time. These events are simply referred to as "the Cataclysm" for short.

Many historical archives and technologies were lost, leaving later archeologists and historians to piece things together. There's a lot of historical disagreement among scholars, so things are a little fuzzy on human history before the Cataclysm. However, this results in a lot of myth and political propaganda of previous time periods merging into history, leaving humanity with an unclear view of itself. It took many generations for humanity to get back on track, though many areas of the world remained unlivable for various reasons.