A person who uses Ninjutsu is a ninja. Ninjutsu is not a martial art. Ninjutsu is an independent art of warfare that developed mainly in the regions of Iga in Mie Prefecture, and Koka in Shiga Prefecture, Japan.
So, do you know what kind of job ninja were doing?
Most people imagine that ninjas flew through the sky and disappeared, like Superman, waving ninja swords around, sneaking into the enemy ranks and assassinating generals... This is a mistaken image of the ninja introduced by movies and comic books.
The jobs of a ninja are divided into the two main categories of performing espionage and strategy. The methodology for performing espionage and strategy is Ninjutsu. Espionage is similar to the job of modern spies, wherein one carefully gathers intelligence about the enemy and analyzes its military strength.
Strategic activities are skills that reduce the enemy’s military power. Ninja did not fight strong enemies by themselves. Ninja fought enemies after they had reduced the enemies’ military power. In times of peace, Ninjutsu was called an art of “entering from afar”, while in times of war, Ninjutsu was called an art of “entering from “nearby”, wherein ninja would constantly gather intelligence concerning the enemy, thinking of ways to beat the enemy, but not fighting the enemy directly. Ninja who thought rationally thought of war by intellect as great, and war by military strength (weapons) as foolish. Therefore, ninja who swing their ninja swords about can be called the lowest of the ninja.
The Ninjutsu of Iga-ryu and Koga-ryu stem from the same source, and are said to be the highest of the Ninjutsu.

From when did Ninja, who worked in the world of shadows, exist? Those roots are found in the “art of warfare” that began around 4000 B.C. in Indian culture, was passed to the Chinese mainland, and around the 6th century, passed through the Korean peninsula and crossed over to Japan.
In that period, a man name Otomono Sahito, who was used by ruler Shotoku Taishi, is said to be the root of the Ninja.
Have you ever heard something like this? "Shotoku Taishi could hear the words of ten people at once." Actually, there is also a theory that he used ninja to obtain intelligence beforehand. However, it is still amazing that Shotoku Taishi could remember all that intelligence...
The continental military strategy that was brought from China was developed in conjunction with shugendo, a practice involving mountain training, and adapted to Japan’s extremely hilly, narrow geography, becoming unique Japanese strategy. From this body of strategy emerged Ninjutsu. There were shugen studios in the Iga and Koka regions. Also, the houses of Todaiji and Kofukuji in the Iga region had most of the country’s warriors, and the lords of these houses adopted guerilla-like tactics, and kept the peace by containing one another. From this, Ninjutsu was developed.

In Iga of the manorial system period, rulers and lords (guardians) did not last long. Because the people of Iga created living areas by manor in units of clans, formed an organized party of landowning farmers, and did not defer to the control of central regimes, an important 12-member council (representatives) was chosen from among the 50-60 members of the party in Iga, and they maintained safety in Iga by cooperation. This is called the “Iga Sokoku Ikki”.

In the 6th year of Tensei (1578), the ruler of Ise, Kitabatake Nobukatsu (the second son of Oda Nobunaga was adopted by Kitabatake and inherited the reins of the family) planned to attack Iga with Maruyama Castle as base, but retreated in the face of an attack from the troops of Iga. In the 7th year of Tensei (1579), Kitabatake Nobukatsu regrouped and again attacked Iga, but was defeated by the resistance of Iga’s troops. Upon hearing of this (the First Tensei Iga War), Oda Nobunaga was sorely angry, and decided to go to battle himself. In the 9th year of Tensei (1581), he led his 50,000 troops to Iga, burning all of its lands and repeatedly slaughtering adults and children alike. The Iga troops resisted to the end, but a compromise was made, and they submitted. This is the only war in which the Iga region was crippled by attack, and the 800-year manorial system of Iga region was finished, and the ninja were scattered among all lands thereafter (Second Tensei Iga War).

The three Iga Ninja Grandmasters

• Hattori Hanzo
• Momochi Tambanokami
• Fujibayashi Nagatonokami

The most famous group of Iga ninja is Hattori, Momochi, and Fujibayashi. Hattori Hanzo, Momochi Tambanokami, and Fujibayashi Nagatonokami are the three Iga Ninja Grandmasters.
Hattori controlled western Iga. There is a famous person who supported Tokugawa Ieyasu, named Hattori Hanzo Masanari. The Hanzo name was inherited.
Momochi controlled southern Iga. The Oe party had originally prospered in the south, and Momochi was one of the supporting families to it, but joining forces with Hattori and riding its wave of strength, Momochi was able to keep its position until the Edo period.
Fujibayashi controlled northeastern Iga. Fujibayashi Yasutake, the author of traditional Ninjutsu text “Mansen Shukai” was of this group.

Among existing traditional Ninjutsu books, “Mansen Shukai”, “Shoninki”, and “Shinobi Hiden” are called the Three Great Books of Ninjutsu.
Many traditional books were written in the Edo Period, and before that traditions were oral. It is assumed that they were written to pass on traditions and commit them to record. The are traditional texts in which the words “there is an oral tradition” stand out, and this may indicate that oral tradition was of greater importance.

• Mansen Shukai, by Fujibayashi Yasutake, integrated Iga and Koka Ninjutsu, and a few types of copies are passed down in both Iga and Koka.
• Shoninki, by Fujibayashi Masatake, is a traditional text of the Kishu-ryu.
• Shinobi Hiden, by Hattori Hanzo is a traditional text of Iga and Koka.

We now use the unified term “ninja”, but did you know that they were not called “ninja” in the past? Depending on the era and region, there were many different ways to call them. Let’s introduce some of those here.

The era

• Asuka Era　－　志能便（Shinobi）
• Nara Era　－　伺見（Ukami）
• Sengoku Era　－　間者（Kanja）・乱破（Rappa）
• Edo Era　－　隠密（Onmitsu）
• Taisho Era　－　忍術者（Ninjyutsusha）・忍者（Ninsya）

The region

• Kyoto ・Nara　－　水破（Suppa）・伺見（Ukami）・奪口（Dakkou）
• Yamanashi　－　透破（Suppa）・透波（Suppa）・三ツの者（Mitsu-no-mono）・出抜（Suppa）
• Niigata・Toyama 　－　軒猿（Nokizaru）・間士（Kanshi）・聞者役（Kikimonoyaku）
• Miyagi　－　黒はばき（Kurohabaki）
• Aomori　－　早道の者（Hayamichi-no-mono）・陰術（Shinobi）
• Kanagawa　－　草（Kusa）・物見（Monomi）・乱破（Rappa）
• Fukui　－　隠忍術（Shinobi）

There are many other names in the different regions, but the above are the most representative.
There are various ways to call ninja, depending on their relation to being secretive, the jobs they performed, and the reading of the Chinese characters with which their names are written.