I find Yagyū Shinkage-ryū interesting because it is one of the few martial schools whose reputation rests as much on what it wrote as on what it did. Plenty of traditions claim a philosophy; this one produced, in Yagyū Munenori's Heihō Kadensho and in the Zen letters Takuan Sōhō sent him, documents that are still read by people who will never pick up a sword. That makes it a good place to think about what happens when a battlefield skill is asked to become a way of living.
The history is, by the standards of sixteenth-century Japan, unusually well attested. Kamiizumi Nobutsuna was a real and celebrated swordsman, and the transmission he granted to Yagyū Munetoshi in 1565 belongs to the documentary record rather than to later legend. I want to be careful here, because koryū histories are often dressed up with undefeated duel counts and miraculous demonstrations, and Yagyū Shinkage-ryū has its share of these. The story of Munetoshi disarming Tokugawa Ieyasu with his bare hands is repeated everywhere; it may well be true in outline, but it has the shape of a founding anecdote, and it is worth holding a little more loosely than the dates and the documents.
What is not in doubt is the family's extraordinary rise. It is hard to overstate how unusual it was for a fencing instructor to become a daimyō and a senior inspector of the shogunate, and Munenori managed both. This is the fact that gives the school's philosophy its edge. When a man with real political power writes that the finest use of the sword is to give life rather than take it, the claim is not the consolation of someone who never had to use violence. It is the considered position of someone who stood very close to it.
I think the katsujinken is easy to sentimentalise and worth reading more strictly than it is usually quoted. Munenori was not a pacifist, and the Heihō Kadensho is not a peace pamphlet. The life-giving sword sits beside the death-dealing sword as its harder counterpart: the skill of ending a conflict without killing is presented as more difficult and higher than the skill of killing, not as a refusal to fight. Read that way, the idea is less comforting and more demanding, and it has more to say to anyone whose work involves the threat of force.
The Zen connection deserves the same care. It is tempting to treat Takuan's Fudōchi Shinmyōroku as proof that the school was "really" a spiritual discipline in disguise. I would put it the other way around. Takuan was writing to a working swordsman about a practical problem: the way a mind that stops and fixes on a single thing, an opponent's blade or one's own fear, leaves the body a step behind. The "immovable wisdom" he describes is immovable precisely because it does not stick anywhere. That is a technical observation about attention before it is a religious one, which is why it has travelled so well beyond its original setting.
The split into the Edo and Owari lines is the part of the story I find most human, and the part most often flattened into a simple question of who was the "real" heir. Munetoshi passed the headship of the school to his grandson Toshiyoshi, not to his son Munenori, and the Owari line is usually treated as the orthodox keeper of the transmission. But Munenori, in the capital, had the shōguns' ear and wrote the book everyone remembers. Both lines were genuine, and treating one as the true school and the other as a footnote tells you more about the teller's loyalties than about the history.
It also matters, for honesty's sake, that the two lines did not fare equally over four centuries. The Owari Yagyū line continues today under a named head in Nagoya, with a traceable succession. The Edo Yagyū family did not maintain the same unbroken family headship, and a good deal of what is practised under Edo-derived labels is continuation and reconstruction rather than direct inheritance. None of this diminishes the art, but it is the kind of distinction the school's admirers tend to blur, and exactly the kind a reference should keep clear.
In the end I value Yagyū Shinkage-ryū less as a set of techniques, most of which I will never see, than as a sustained argument about the relationship between skill and restraint. It was made by people who had the power to be careless with violence and chose instead to think hard about not being. That argument outlived the world that produced it, which is probably the best evidence that there was something real in it.