OHHH, although on second reading I will retract my "assassins" label as they were involved in many sorts of covert operations, however it was the assassination role that seemed to birth the need for the "ninja." Intelligence and information gathering personnel have existed since the time of Sun Tzu so I inferred that it was the taboo and honourless act of assassination that garnered the need for them as a whole (even though their skills were many and varied far beyond that)
train how you fight, fight to live.