A or was a covert agent

Covert agent

-Covert agents in the United States:As it is used in the United States Intelligence Community, it is legally defined in 50 USCA §426.The definition is subject to judicial interpretation, but a reading of the plain language of that statute reveals that a covert agent can be an employee of the US...

 or mercenary

Mercenary

A mercenary, is a person who takes part in an armed conflict based on the promise of material compensation rather than having a direct interest in, or a legal obligation to, the conflict itself. A non-conscript professional member of a regular army is not considered to be a mercenary although he...

 of feudal Japan specializing in unorthodox arts of war. The functions of the ninja included espionage

Espionage

Espionage or spying involves an individual obtaining information that is considered secret or confidential without the permission of the holder of the information. Espionage is inherently clandestine, lest the legitimate holder of the information change plans or take other countermeasures once it...

, sabotage

Sabotage

Sabotage is a deliberate action aimed at weakening another entity through subversion, obstruction, disruption, or destruction. In a workplace setting, sabotage is the conscious withdrawal of efficiency generally directed at causing some change in workplace conditions. One who engages in sabotage is...

, infiltration

Infiltration tactics

In warfare, infiltration tactics involve small, lightly equipped infantry forces attacking enemy rear areas while bypassing enemy front line strongpoints and isolating them for attack by follow-up troops with heavier weapons.-Development during World War I:...

, and assassination

Assassination

To carry out an assassination is "to murder by a sudden and/or secret attack, often for political reasons." Alternatively, assassination may be defined as "the act of deliberately killing someone, especially a public figure, usually for hire or for political reasons."An assassination may be...

, as well as open combat in certain situations. The ninja, using covert methods of waging war, were contrasted with the samurai

Samurai

is the term for the military nobility of pre-industrial Japan. According to translator William Scott Wilson: "In Chinese, the character 侍 was originally a verb meaning to wait upon or accompany a person in the upper ranks of society, and this is also true of the original term in Japanese, saburau...

, who had strict rules about honor and combat.

The shinobi proper, as a specially trained group of spies and mercenaries, appear in the Sengoku

Sengoku period

The or Warring States period in Japanese history was a time of social upheaval, political intrigue, and nearly constant military conflict that lasted roughly from the middle of the 15th century to the beginning of the 17th century. The name "Sengoku" was adopted by Japanese historians in reference...

 or "warring states" period, in the 15th century, but antecedents may have existed in the 14th century, and possibly even the 12th century (Heian

Heian period

The is the last division of classical Japanese history, running from 794 to 1185. The period is named after the capital city of Heian-kyō, or modern Kyōto. It is the period in Japanese history when Buddhism, Taoism and other Chinese influences were at their height...

 or early Kamakura

Kamakura period

The is a period of Japanese history that marks the governance by the Kamakura Shogunate, officially established in 1192 in Kamakura by the first shogun Minamoto no Yoritomo....

 era).

In the unrest of the Sengoku period (15th–17th centuries), mercenaries and spies for hire arose out of the Iga

Iga Province

was an old province of Japan in the area that is today western Mie Prefecture. It was sometimes called . Iga bordered on Ise, Ōmi, Yamato, and Yamashiro Provinces.-Geography:...

 and Kōga

Koka, Shiga

is a city located in the southern part of Shiga Prefecture, Japan....

 regions of Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

, and it is from these clans that much of later knowledge regarding the ninja is inferred.