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There are, depending on which reliable authorities you talk to, either no authentic ninjutsu traditions left in the world, or one extant authentic tradition, passed through Takamatsu Toshitsugu. Without getting into all the details here, I think there's plenty of evidence -- both internal and external -- to say that Takamatsu passed authentic ninjutsu techniques and teachings to his students, including his official successor, Masaaki Hatsumi.
But that hasn't stopped plenty of others from claiming ninja mastery (sorry, Tommy).
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I think most martial arts enthusiasts have had a passing familairity with
Ashida Kim, the most famous
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of all those who claim to be ninja grandmasters. His books are fairly laughable on the face, mixing Japanese history with Chinese kung fu terminology. Check out his website, in which he lays down a challenge to all those who doubt him. He will fight them anywhere, any time. All you have to do is fly him out, and pay him $10,000 up front -- non-refundable. You know what, I too will take all bona fide offers, but my fee is $100 Million up front, non-refundable even if I lose -- oh, plus any medical expenses. Why sell myself low?
There's also
Konigun Ninjutsu. These guys are a "Christian" ninja group. I'm not exactly sure why that's important. This group too also seems to mix up their
hoplogical history and mix Okinawan Kobudo weapons with Japanese weapons of the feudal period. Okinawan fighting systems are not the same as Japanese fighting systems. Neither samurai or ninja used sai -- I don't care what you see the Turtles using. Also, the Konigun rank system is a little funky; "Sashes" with titles like "bushi" and "samurai?" It's such an afront to Japanese martial history, I just don't know where to start complaining.
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Finally there's
Nindo Ryu; which at least doesn't pretend to be a true ninjutsu system. They are near the front of the pack of so-called "gendai ninjutsu" groups which claim to be "inspired by" traditional ninjutsu. These groups take other hand-to-hand combat systems and weapons systems and add a little combat/wilderness survival training, and who-knows what else to come up with a curriculum.
Ninjutsu is not just "karate plus (fill in blank)." Understanding that fact isn't easy for some people. Americans are part of a young country with a history of breaking from traditions to forge new ones. The current American culture is a robust juggernaut wearing blinders. It's sometimes hard for us to accept the sense, the wonder, and the use of tradition. By nature, we believe we can do better.
I don't want to come off as "holier than thou" on this topic. But I do think that the arrogance of others can sometimes illuminate the traps and pitfalls we all face on our paths.