Chapter 1: Yori

It is the beginning of the "BW", or the before walls period. Suddenly there are these creatures popping up, eating people. There is a shift in society, suddenly the government becomes more secretive and alchemists are being taken away. The world is changing...
As these monsters are rapidly growing in numbers, the people are getting more tense, as the home they once new, has turned into a living hell.

cover source:https://www.reddit.com/r/FanTheories/comments/26rkde/aot_where_does_aot_take_place/

Note: I don't not own the AOT franchise or the concept of it, this is just my take on the history of the AOT universe before the titans started popping up.