(www.entertheninja.com)The history of the Ninja is hard to trace. This is because there were few, if any records kept of its existance. Most of what is known about the ninja is taken from stories that have been passed down from generation to generation.
Nonuse ("the art of stealth") was first introduced to Japan in 522 A.D. as a religion practiced by priests. These priests were not violent people, they were "mystics" who gathered and shared information for the ruling classes. The ninja as we know them were not introduced until later.
It wasn't until 645 A.D. that the priests perfected their fighting skills and made use of their knowledge of nonuse. This was because they found themselves being harassed by the central government and found in necessary to protect themselves.
In 794-1192 A.D. the new civilization flourished and with it, a new class of wealthy, privileged families. These families fought with one another in attempts to make or destroy emperors. The need for spies, informants and now assassins grew as these families dueled for power. They were suspicious and jealous of one another and would resort to any means necessary to eliminate any possible threats. Therefore, the practitioners of nonuse were in great demand. With this, the ninja was born.