Ninja (忍者) were said to be agents of espionage and assassination in feudal Japan in legend and popular fiction. Some modern practitioners of budo ninjutsu argue that ninja were used primarily as spies, not assassins. It is popularly believed that the ancient ninja were peasants, forbidden under law from studying the samurai swordplay techniques because of feudal Japan's caste structure. Outside of the modern schools teaching ninjutsu, there is little verifiable evidence that ninja existed as such in feudal times, at least in the form known from modern movies, videogaming and popular fiction. Because of their obscure history, little verifiable information on ninja exist.

Ninja are said to have made use of weapons that could be easily concealed or disguised as common tools. Weapons commonly attributed to them included shuriken and bo. In popular folklore, ninja also used special short swords called ninja-ken (or ninja-to see below for explanation). Ninja-ken are smaller than katana but larger than wakizashi. The ninja-to was more of a utilitarian tool than a weapon. Another version of the ninja sword was the shikoro ken (saw sword). The shikoro ken was said to be used to gain entry into fortresses. The shikoro ken supposedly could also be used to cut (or saw) through opponents.

Etymology

Ninja is a Sino-Japanese compound derived from the two kanji (Chinese characters) used to write shinobi-no-mono, one of the native Japanese words for people who practice ninjutsu (sometimes erroneously called ninjitsu). Ninja and shinobi-no-mono, along with shinobi, another variant, became popular in the post-World War II period through the works of authors like Tomoyoshi Murayama, Sampei Shirato, and Ryotaro Shiba; before the war, the expressions ninjutsu-zukai and yōjutsu-zukai were more common. The word shinobi itself, written phonetically with the kanji 志能備, has been traced as far back as Japan's Asuka period, when Prince Shotoku is alleged to have employed one of his retainers as a ninja.

The underlying connotation of shinobi (忍, pronounced nin in Sino-Japanese compounds) is "to do quietly" or "to do so as not to be perceived by others" and—by extension—"to forebear," hence its association with stealth and invisibility. Mono (者, likewise pronounced sha or ja) means "person." The nin of ninjutsu is the same as that in ninja, whereas jutsu (術) means skill or art, so ninjutsu means "the skill of going unperceived" or "the art of stealth"; hence, ninja and shinobi-no-mono (as well as shinobi) may be translated as "one skilled in the art of stealth." Similarly, the pre-war word ninjutsu-zukai means "one who uses the art of remaining unperceived."

History

Because ninja rarely left anything in writing or boasted of their achievements, the history of the ninja is mostly apocryphal and blatantly legendary, so the great majority of stories circulating about them are difficult to prove.

Historical examples of ninja are said to include Minamoto no Yoshitsune, who employed surprise as a major weapon in his victories. He is said in a popular folktale to have been educated by a tengu to learn tactics and became a ninja.

One of the earliest roots of ninja, Togakure Ryū, reportedly originated in the late Heian Period. Iga and Kōga are two of the most famous ninja styles, and are often pitted against each other in fiction. Some also say that they were allies and worked together in mutual defense pacts. Both of these claim that they originated in the Heian period.

Only a few records remain from the Kamakura period. Kusunoki Masashige used tactics against enemies that remotely resemble ninja tactics. From the Muromachi period there are even fewer records. Both of these times were generally peaceful, and many battles had tournament-like aspects that prevented surprise attacks. Somewhere in these time periods, bushido began to form as the proper and honorable way a samurai must follow. It would be well into the Edo period that bushido was finally formalized.

In the Sengoku Period, also known as the Warring States period, ninja supposedly flourished. Almost all famous daimyō are said by modern ninja schools to have had ninja, or a ninja-like group under his control and they served as eyes and ears. Some daimyō were reportedly ninja themselves. The clan of Sanada, the most famous member being Sanada Yukimura, was reportedly a ninja clan. This is attributed to the successful defense of their castle with approximately 3,000 soldiers against an overwhelming force of 50,000 led by Tokugawa Hidetada. Their tactics, which are said to have included splitting their house in two, one each supporting Toyotomi and Tokugawa in order to survive no matter which side finally won, has been the subject of much legendary discussion. Later, they would come to be called Sanada Jū Yushi, lit. Ten heroes under Sanada, in tales where they used ninja skills to defeat everything but their jealous wives (who were, of course, ninja themselves).

Tokugawa Ieyasu was claimed to have used ninja, controlling both Iga and Koga in unifying Japan and ultimately rising to the rank of Shogun. In his dramatic escape through the mountainous landscape of Nara after Oda's assassination, Iga ninja led by Hattori Hanzō allegedly helped Ieyasu escape, gaining his favor. The last battle where ninja reportedly fought is in the Siege of Shimabara under the Tokugawa shogunate. As the shogunate became stable, ninja were effectively unemployed. Some were said to have become Oniwabanshū, a semi-secret group of bodyguards and intelligence officers who worked tending gardens of the Edo castle and eavesdropping on the daimyō. A purported ninja master Fujibayashi Sabuji wrote Bansenshukai (万川集海) as a collection of ninja knowledge. Yet the stories go that most knowledge was still passed on through oral tradition and training, as most ninja believed that their services would soon be needed again. The peace of the Edo period continued for over 200 years.

In the Edo period, ninja became popular heroes in books and plays. Many mythical ninja powers such as becoming invisible, jumping over tall fences, casting spells and calling up a giant toad larger than a human were all invented in these fictitious accounts of ninja. Ninja did not correct these misconceptions and some may have even written these stories themselves to increase their value should their services be needed. One of the lesser known contributions made by ninja is their involvement in furthering the research of fireworks as a result of their development of pyrotechnic weaponry.

Some say ninja were a tribe of assassins below the samurai caste. Others say the Ninja were an elite group of Samurai trained for Assassination and spying missions. The latter is more likely as there are records of samurai who knew Ninjutsu although this contradicts the traditional code of honour. Ninjas were not known for holding a high code of honour.

In the 1900s,the Yamabushi monks knew some ancient secrets and were believed to be ninjas.