In Japanese culture, ninjas were assassins, scouts, saboteurs and spies who appeared in fourteenth century feudal Japan when clan warfare was rampant.
Ninjas were trained for dangerous and high-risk stealth missions very similar to those of the modern day special forces.
Because the Samurai Warrior Code (Bushido) forbade such tactics as dishonorable, ninjas were employed in guerrilla warfare in cases where they wanted to avoid all out war.