Mass media ninja inflation has debased the currency of the legendary ninja assassin. Are there any films out there that depict how historical ninjas actually were trained, what skills they had, how proficient in battle they were, and their proper context?

Well there are plenty, but their well hidden in plain sight and kinda blend in with everything. Not a very high budget thing very often.
Sorta like real ninjas.

As far as I know, it's hard to even find reliable historical information about ninja. It's kind of like finding accurate historical information about Robin Hood or King Arthur.

In Japan, they have "ninja villages" which purport to be historical reenactments, but lean hard into the legends of ninja gadgetry and secret techniques. They're apparently closer to the RenFaire than to Colonial Williamsburg.

Historically, ninja/shinobi were the commandos of their day, irregular troops who operated out of uniform. They did a lot of scouting, spying, and sabotage, probably more so than assassinations.

I don't have any good historical sources for you, nor any suggestions for accurate movies. The last ninja movie I watched was Ninja III: The Domination, which is short on historical accuracy, and long on Sho Koshugi and Lucinda Dickey of Breakin' fame.

Your best bet might be to start with historical sources you know to be fairly reliable, and see if any of them have touched on the subject.

Ninja III is a damn good movie, though Sho Kasugi only shows up in the last 1/3 or so.
 
But in that vein, I'd say The Executioner has Sonny Chiba playing a fairly realistic ninja in a modern (70s) setting.

 Shogun has a pretty realistic depiction.  Though there is no depiction of their training, they raid the protagonist and his benefactor a couple of times.  They are presented as using surprise, decent in combat and getting wrecked badly once Samurai have a sword out and it becomes a straight fight.

The Japanese ninja movies I've seen are about as realistic as Chinese wuxia films, i.e. they're pure fantasy.

Quote from: JeremyR on June 21, 2023, 01:58:27 AM
Ninja III is a damn good movie, though Sho Kasugi only shows up in the last 1/3 or so.

I don't know if it's good, but I found it enjoyable.

I wouldn't say it compares to an actually good martial arts movie like The Chinese Connection, or Iron Monkey, or 36th Chamber of Shaolin.

Sanada Yukimura No Bouryaku 1979. Goes by a few titles like Renegade Ninja and Shoguns Assassins.

Some fantastical elements but otherwise played mostly straightforward.

https://www.amazon.com/Book-Ninja-Bansenshukai-Japans-Premier-ebook/dp/B00F8EYWES/

Not a movie, but this book is purportedly a translation of a 17th century ninja manual.

https://www.amazon.com/Burglars-Guide-City-Geoff-Manaugh-ebook/dp/B00V35U0TM/

I've also been reading this, a discussion of the thoughts and techniques of professional burglars who do things like using an architecture degree to plan heists.

Quote from: oggsmash on June 21, 2023, 05:36:13 AM
Shogun has a pretty realistic depiction.  Though there is no depiction of their training, they raid the protagonist and his benefactor a couple of times.  They are presented as using surprise, decent in combat and getting wrecked badly once Samurai have a sword out and it becomes a straight fight.

I second this one. It's not over-the-top or anything like that. That series is available in various places. Some of Clavell's other novels, chiefly Gai Jin, also contain some info in ninjas (never made into series or films).

Quote from: KrisSnow on June 25, 2023, 06:16:14 PM
https://www.amazon.com/Book-Ninja-Bansenshukai-Japans-Premier-ebook/dp/B00F8EYWES/

Not a movie, but this book is purportedly a translation of a 17th century ninja manual.

https://www.amazon.com/Burglars-Guide-City-Geoff-Manaugh-ebook/dp/B00V35U0TM/

I've also been reading this, a discussion of the thoughts and techniques of professional burglars who do things like using an architecture degree to plan heists.

This guy's a bit sketchy as he doesn't actually read Japanese, but has a friend do the translations for him.  The definitive book on the mythologization of ninja is this one: https://www.amazon.com/Ninja-Unmasking-Myth-Stephen-Turnbull/dp/1473850428/ref=sr_1_1?crid=QPV93G0N9HYU&keywords=ninja+unmasking+the+myth&qid=1688225584&s=books&sprefix=ninja%3A+unmasking+%2Cstripbooks%2C118&sr=1-1

What's especially interesting is that the author admits that he bought into and helped perpetuate some of the ninja myths but now he wants to set the record straight using actual historical sources and tracing the evolution of ninja representation through Japanese history.  The result is pretty compelling and I'm a historian of Asia myself so I can appreciate the work he does here.  Incidentally, he references those translated manuals, but avoids casting too much shade out of professional courtesy I suspect.

But the short answer is that your picture of "ninja/shinobi" is likely to be about 85% incorrect and manufactured by Japanese & foreign pop culture and media.

It's worth mentioning that the existence of ninja is a historical fact. History books mention Oda Nobunaga employing them during the Sengoku period, for instance.

it's the details that are often sketchy.