Last week we told the story of Yagyū Shinkage-ryū becoming one of two official sword schools of the Tokugawa shogunate. This week we cover the other one — and arguably the one with the deeper fingerprint on the martial art practiced in dojos around the world today. If you have ever picked up a shinai, put on bōgu armor, or thrown a strike at an opponent’s men, kote, or dō, you are touching the legacy of Ittō-ryū, whether you know it or not.

Its founding principle is contained entirely in its name: ittō, one sword. The belief that all of swordsmanship — every parry, every feint, every footwork pattern — ultimately resolves into a single, decisive cut.

Itō Ittōsai Kagehisa was born around 1560, by tradition on the remote Izu Ōshima Island. His childhood alone reads like legend: he reportedly left home at 14, survived a crossing of Sagami Bay on driftwood, and fought off bandits along the way — formative experiences in a famously self-reliant fighting style. He went on to study Chūjō-ryū kenjutsu in Kamakura under Kanemaki Jisai, a student of the swordsman Toda Seigen, before mastering its techniques and setting out on his own musha shugyō.

What followed was one of the most extreme combat records of any figure in this series: an estimated 33 duels to the death across Japan, all of them survived. From that crucible, sometime in the late 16th century, he distilled everything into Ittō-ryū — a system built around the conviction that genuine mastery doesn’t need a hundred techniques. It needs one cut, executed with total unity of mind, body, and blade.

Mikogami Tenzen won. He was so grieved at having killed his friend and training partner that he adopted the surname Ono in his honor, becoming known thereafter as Ono Jirōemon Tadaaki. That single, brutal succession moment became the foundation of Ono-ha Ittō-ryū — the senior, most influential branch of everything that followed.

Tadaaki’s skill brought him into direct service of the Tokugawa. He was named, alongside Yagyū Munenori, as one of two official kenjutsu instructors to shogun Tokugawa Ieyasu — and he went on to teach Ieyasu’s son Hidetada and grandson Iemitsu as well. For the better part of the Edo period, Ono-ha Ittō-ryū and Yagyū Shinkage-ryū stood as the twin pillars of shogunal swordsmanship — sometimes framed as a contrast in approach: Ittō-ryū the art of decisive technique, Yagyū Shinkage-ryū the art of strategy.

Ittō-ryū’s most lasting legacy doesn’t come from its founder or even from Ono-ha directly — it comes from a later branch, Nakanishi-ha Ittō-ryū, founded by Nakanishi Chūta Tanesada in the mid-18th century. To understand why this matters, you need one piece of context from outside the Ittō-ryū lineage: in the early 1700s, a swordsman named Naganuma Shirōzaemon Kunisato of Jikishin Kage-ryū had pioneered the first standardized set of bōgu — protective armor — paired with a flexible practice sword, allowing students to strike at full power without fear of seriously injuring each other.

Nakanishi-ha’s adoption of shinai-and-bōgu sparring transformed Ittō-ryū’s popularity. After two centuries of enforced Tokugawa peace, with no real battles left to fight, young samurai were drawn to a training method that finally let them compete at full intensity, safely. Ji-geiko — free sparring — was born from exactly this innovation, and it is the direct ancestor of every kendō match fought today.

Ittō-ryū fractured into numerous lineages over the centuries, each adapting the founder’s one-cut philosophy to its own region and era:

• Ono-ha Ittō-ryū — the senior branch, founded by Ono Tadaaki, continuing today under the 18th headmaster at the Reigakudō dojo in Tokyo
• Nakanishi-ha Ittō-ryū — refined the shinai and pioneered bōgu sparring within Ittō-ryū, profoundly shaping kendō’s tactics and aesthetic
• Hokushin Ittō-ryū — created late in the Tokugawa era, leaned heavily into shinai sparring, and several of its senior exponents helped formally establish modern kendō during the Meiji period
• Mizoguchi-ha Ittō-ryū — adopted as the official school of the Aizu clan, still maintained today by Fukushima prefectural kendo federations
• Kogen Ittō-ryū — founded by Henmi Tashiro Yoshitoshi; its original dojo still stands in Saitama Prefecture

• One of only two sword schools officially patronized by the Tokugawa shogunate, alongside Yagyū Shinkage-ryū
• Kiriotoshi — Ittō-ryū’s signature cutting-down technique that seizes the centerline — directly informs modern kendō’s debana waza, timing-based counter strikes
• Nakanishi-ha’s adoption of shinai and bōgu sparring became the technical and cultural foundation that modern kendō was built on
• Multiple Ittō-ryū exponents sat on the committees that formally established the modern kendō curriculum during the Meiji period
• Yamaoka Tesshū — one of the 19th century’s most celebrated swordsmen, Zen practitioners, and calligraphers — trained extensively in Ono-ha and Nakanishi-ha Ittō-ryū
• Ono-ha Ittō-ryū remains active today at the Reigakudō dojo in Setagaya, Tokyo, under its 18th-generation headmaster

If Katori Shintō-ryū is the deepest root of Japanese swordsmanship and Yagyū Shinkage-ryū is its philosophical conscience, Ittō-ryū is its most direct technical ancestor in the modern sense. Every time a kendōka steps onto the floor in bōgu armor and exchanges full-speed strikes with a shinai, they are participating in a training revolution that began inside this single lineage.

Itō Ittōsai built his school on the idea that everything reduces to one true cut. Two centuries later, his descendants — almost by accident, while looking for a safer way to train — built the foundation of a sport now practiced by millions of people worldwide who have likely never heard his name.

• Wikipedia — Ittō-ryū — en.wikipedia.org/wiki/Ittō-ryū. Core lineage history, branch details, and the Nakanishi-ha shinai/bōgu development.
• Wikipedia — Shinai — en.wikipedia.org/wiki/Shinai. Confirms the distinct attribution of early bamboo training swords (Kamiizumi) versus the modern four-slat design (Nakanishi Chūzō Tsugutate).
• Caile, Christopher — “Itto-ryu Kenjutsu: An Overview,” fightingarts.com. Detailed lineage breakdown of Ono-ha, Nakanishi-ha, Hokushin, and Mizoguchi-ha branches.
• Seishin-Kan — “Ono-Ha Ittō-Ryū Kenjutsu,” seishin-kan.org/itto-ryu.html. Account of the founding duel between Ono Zenki and Mikogami Tenzen.
• Sasamori, Junzo; trans. Mark Hague — Secrets of Itto-ryu, Book One (2023). English-language history of Ittōsai and Ono Tadaaki by a 16th-generation Ono-ha headmaster’s lineage.
• RKC Dojo — “Introduction to Kendo, The Art of The Sword,” rkcdojo.com. Clarifies the separate roles of Naganuma Kunisato (bōgu) and Nakanishi Chūzō Tsugutake (shinai) in kendō’s development.
• Martial Arts Preservation Association — martialartspreservation.com/history. Background on Ono Tadaaki’s appointment alongside Yagyū Munenori as official Tokugawa instructors.