A ninja (忍者 ninja?) is a highly skilled operator, specially trained in the art and science of assassination, espionage, guerilla warfare, covert killing, and combative martial arts. In Japanese culture, they are usually trained for dangerous and high-risk stealth clandestine missions very similar to those of the modern day special forces like the US Navy Seals or British SAS. Although their exact origins are still unknown, with some historians speculating about some Chinese origin or influence, it is known that they appeared in fourteenth century feudal Japan, and remained active from the Kamakura to the Edo period. Their roles may have included sabotage, espionage, scouting and assassination missions as a way to destabilize and cause social chaos in enemy territory or against an opposing ruler, perhaps in the service of their feudal rulers (daimyo, shogun), or an underground ninja organization waging guerilla warfare.