In 1588 the powerful Toyotomi Hideyoshi set upon Japanese society the “sword hunt” edict, which mandated the requisitioning of swords and armor from the common people. Hideyoshi, using religious allusions to impel them to do their part in bringing a long peace to Japan, announced that the metal from the confiscated swords would be contributed to the creation of a monolithic statue of Buddha, thus implying the commoner could absolve a bit of negative karma by following the order (Mason & Caiger 1997 p179). This campaign to disarm the population proved successful, and from then on a clear distinction between nobility and the peasantry was evinced. Samurai could carry the sword, while farmers and the lowly could not.
It was a time of great transition. The blood feuds of the Sengoku Jidaii were fading into history, foreigners were being expelled from the country in preservation of culture, and the ways of the shinobi were in senescent decline (like Fall leaves descending into the dim obscurity of a forgotten season).
With the artificial peace of the Edo period well under way, Kiumura Okunosuke Yasutaka, sensei of the Koka Ryu traditions, imparted his predictions of the fate of his ninjutsu to Chikamatsu Shigenori. The conversation entailing these predictions was recorded in the Koka Shinobi no Den Miraiki (1719 AD).
In this document, Kimura portends that the younger generations lineally tied to the Koka would be lulled, by the peace of Edo, into the comforts of an easier life, hence those cultivating skill in ninjutsu would continue to decline in number. Peace was making people soft and complacent while the utility of ninjutsu was being forgotten in the absence of war.
Kimura doesn’t stop there however. He references the vetting process involved in imparting the most secret traditions of Koka ninjutsu to the aspiring student, stating how those running the schools in his time possessed only a partial knowledge of deep ninjutsu. He explains how ninjutsu is ‘indispensable’ to the efforts of warfare, and thereby implies the sanctity of the art and why it must be preserved.
This sensei of the Koka ninja and Chikamatsu Shigenori responded to the peace of Edo by preserving the myriad secrets of the shinobi for future generations. Peace does not last forever, and for this reason those dark implements of war are best left in the attic than relegated to the burn pile.
Mason, H.P. & Caiger, J. G. (1997). A History of Japan. p179