Excerpted from: Mitchell, David. 1989. The Complete Book of Martial Arts. Hamlyn Publishing. Under the feudal military government of Japan, known as the Tokugawa Shogunate, overt acts of warfare between rival lords were frowned on. The climate was there­fore ripe for using mercenaries who, if they were caught, could not be traced back to any particular employer.

The idea of covert warriors, as distinct from mercenaries, was not a new one. Feudal lords had long appreciated the value of...