Justice, the only Gear able to command the legions of biological weapons, went berserk and declared war on all of humanity. That was the beginning of the worst war in history - The Crusades.
In response, mankind formed an order of elite knights, and with their participation, Justice was defeated. With her fall, all Gears ceased to function and entered a dormant state. The long-lasted Crusades came to an end, and it seemed like mankind restored peace.
However, true history has yet to reveal itself.
Guilty Gear 2: Overture Features: