Establishment of Ninjutsu Books
During the Warring States period, the Iga and Koga ninja lent their strength as soldiers to the warring feudal lords and were a group that excelled at surprise attacks such as night attacks and arson in many famous wars. In the peaceful Edo period, they began to study the past wars that took place in the Warring States period.
This led to the creation of the “兵法書 Heihō sho,” which summarized the techniques and theories of warfare in the Warring States period. As one of these military arts, the so-called “忍術書 Ninjutsu sho”（Ninjutsu books）were compiled specifically for the techniques that the ninja of the Iga and Koga were good at.
Contents of the Ninjutsu Books
The ninjutsu books mainly contain information gathering, spying, disguising oneself when traveling, and other ninjutsu that are performed while appearing during the day, as well as methods of infiltrating enemy castles and residences and the tools used to do so, and ninjutsu that are performed while hiding in the dark of night, such as surprise attacks at night.
Some of the ninjutsu techniques are spellbinding and unbelievable today, such as steaming a dog’s heart black and holding it in one’s hand to make one invisible.
There are many things that are still unknown, such as when these ninjutsu books were written and whether the ninjutsu described in them can really be used.
Research is being conducted to find ninjutsu books that have not yet been found, to decipher ninjutsu books, and to experiment to realize what is written in ninjutsu books.
The more deeply we study ninjutsu books, the more we will be able to understand the true nature of the ninja. （Takamasa Fukushima）
