The land has been overrun by man-eating monsters called Titans. Larger Titans can level ordinary buildings with ease, and even smaller ones can easily kill large numbers of humans in a short time span. All Titans hunt humans obsessively, and can be effectively fought only with specialized training and equipment.
A community of human survivors remains within three concentric rings of massive walls, which are mostly Titan-proof. The world outside the walls remains almost completely mysterious to them: they don’t know where the Titans come from, whether any other human communities exist, or so on.
The armed forces of humanity are divided into three corps:
- The Military Police, who maintain order within the walls
- The Garrison, who maintain and guard the walls themselves (including manning the emplaced weapons to fend off Titan attacks)
- The Survey Corps, who range beyond the walls
The Survey Corps are the smallest by a significant margin, with correspondingly low overall budget. They regularly suffer death rates of 30% per mission.
(This debate takes place before Episode 1 of the show.)