A ninja or shinobi (忍者 or 忍び?) was a covert agent or mercenary of feudal Japan specializing in unorthodox arts of war. The functions of the ninja included espionage, sabotage, infiltration, and assassination, as well as open combat in certain situations. The ninja, using covert methods of waging war, were contrasted with the samurai, who had strict rules about honor and combat.
In his Buke Myōmokushō, military historian Hanawa Hokinoichi writes of the ninja:
“ They travelled in disguise to other territories to judge the situation of...