Star Trek IV is not what I’m looking for. That was people from the future going back to a less technological past. What i’m looking for is the opposite, people from a lesser tech past (usually the time of whenever the author is writing) going to a higher tech future. And then using tactics that have been forgotten in that future. Of course it doesn’t have to be a literal past->future; it’s low tech->high tech that’s the real important part.
David Brin started a young-adult series in which children from the past were brought to the future - and in some cases, their knowledge of tactics that had been forgotten in the interim was useful; here’s a link to the first one (written by Kress) https://www.goodreads.com/en/book/show/101892.Yanked_
Somewhat similar is Fred Pohl’s “The Wizards of Pung’s Corners,” where the government tries to bring the title town back into the US and where their high tech military is defeated by the villagers using old-fashioned rifles because the don’t break down as easily.
Then again, they take that whale expert with them and the whales. Who better to take care of the whales?
The Roger Corman classic Teenage Caveman. Robert Vaughn goes on a quest and rediscovers the idea for the bow and arrow when he sees a branch snap back.
Turns out, they are from our future after a nuclear war!
Poul Anderson’s “The High Crusade” features knights from medieval Europe doing quite well against advanced aliens because of use of antiquated weapons and techniques.
Harry Turtledove’s WorldWar series has humans defeating aliens by using obsolete techniques as well (a giant cannonball can’t be disabled by lasers like a warhead can).
In Bill and Ted’s Excellent Adventure, Genghis Khan combines ancient cavalry tactics with modern equipment. This equipment–clubs, armor, and even a portable “wheeled horse”–is similar to but much more effective than what was available in Khan’s time. Mall security is largely unprepared for these forgotten but highly effective tactics and only wins through sheer force of numbers.
Not quite what you are looking for but Alita: Battle Angel led me to https://tvtropes.org/pmwiki/pmwiki.php/Main/ArchaicWeaponForAnAdvancedAge
Yes, the same tactics that worked 700 years ago in China were also effective in Oshman’s Sporting Goods.
The key to Springfield has always been Elm Street. The Greeks knew it. The Carthaginians knew it. Now you know it.
There’s a similar storyline in Stargate: SG-1. The super-high tech Asgardians need help from the “dumb” humans, because our low-tech weapons are the only thing that can stop the replicators that have been wiping out the Asgard.