It was the attempt of scientists to technologically and magically modify living creatures into weapons with great destructive potential. One Command Type Gear was made to control all the others, and told by A Country to fight according to their will. It...She...refused and instead declared a complete war on humanity. She was eventually defeated and trapped, so that all the Gears which she commanded went inactive -- they never had any will of their own once they became Gears. They have not all been destroyed, and recreating or restarting the Gear project would likely result in the destruction of humanity. 

That hasn't stopped people from trying.