What draws me to Ittō-ryū is that it is built around a single idea, and that the idea is a good one. Most sword schools accumulate forms, each answering a particular situation. Ittō-ryū does the opposite: it insists that the whole of swordsmanship can be expressed in one cut, kiriotoshi, in which meeting the opponent and defeating him are the same motion. Whether or not that is literally true, it is a remarkable thing to organise an entire art around, and it gives the school an intellectual cleanliness that I find rare.
I have to be honest about the founder, because the school's admirers usually are not. Itō Ittōsai is barely a historical figure at all. The stories about him, the island birth, the youthful ordeals, the dozens of duels won without a scratch, are the standard furniture of the koryū origin tale, and they cannot be checked. I do not say this to debunk him; founders of this period are often shadowy, and a legend can sit on top of a real person. But a reference should be clear about where the legend ends, and with Ittōsai it ends very early. The solid history of Ittō-ryū begins with the second generation, not the first.
That second generation is where the school becomes interesting to me as history rather than folklore. The tale of Ittōsai forcing his two best students to duel for the succession, with the winner taking the loser's surname out of grief, is itself probably embroidered. But Ono Tadaaki is a real, documented man who taught the second Tokugawa shōgun, and that fact places Ittō-ryū, beside Yagyū Shinkage-ryū, at the very centre of the new order the Tokugawa were building. Swordsmanship in Edo Japan was not only a fighting skill; it was a route to status and to the ear of power, and the two schools that secured that position shaped much of what followed.
The part of the story I think matters most, and that is most often undersold, is what Ittō-ryū did to the practice of swordsmanship itself. When the Nakanishi line took up the bamboo shinai and the padded armour, it changed what training could be. For the first time students could strike each other as hard as they liked and walk away, which meant they could test technique against a resisting opponent rather than rehearse it in the air. That single change is the seed of modern kendō. When people put on bōgu today, they are, whether they know it or not, the heirs of an Ittō-ryū workshop.
I am wary of the way this lineage is sometimes used to claim that kendō simply is Ittō-ryū, or that Ittō-ryū is therefore the "true" sword art. Kendō became its own thing, with its own rules and its own purposes, and the koryū that fed it are not reducible to it. What I will say is that no other school has a better claim to have set the modern discipline in motion, and that this is a more interesting kind of importance than the duel-count legends the founder is usually sold on.
In the end I value Ittō-ryū as a demonstration that an art can be both deeply technical and conceptually simple. It took one motion and asked what would happen if everything were understood through it. The answer turned out to be: a great deal, including, in time, the way most of the world now meets the Japanese sword. That is a long reach for a single idea, and it is the reason the school deserves to be remembered for the idea rather than for the myths around its founder.