Gentle also wrote GRUNTS, which I understand involved Tolkien Orcs getting access to modern military hardware; I’ve never tracked down a copy to read myself, but it could be a possibility.
Also Stirling’s Domination of the Draka series, if you have a high tolerance threshold for . . . certain things . . . Anyway, there are countless thrilling battle-scenes in Marching Through Georgia (Draka vs. the Waffen-SS in Caucasian Georgia).
Pournelle, BTW, has edited a series of paperback military-SF anthologies, There Will Be War. Nine or ten volumes, I believe.
Also the three-volume anthology Imperial Stars (one volume of which, of some non-military interest, includes his first published explication of the Pournelle Chart of political ideologies; provides an interesting comparison to the Libertarians’ Nolan Chart).
And, within his CoDominium/Empire of Man universe, used here as a shared universe with multiple authors contributing, the Warworld anthology series, all set on the planet Haven (which might or might not be the same planet as Prince Samual’s World in King David’s Spaceship, which includes a Kingdom of Haven – I’m not clear on that point). Look here if you want to see the Sauron Supermen in battle – and at home.
Or, you could just rent the movie.
That movie is EPIC!
Can you count suckas!?
It isn’t the same planet. Haven is a common name in Pournelle’s universes.
Seconding Grunts. Orc Marines are a hoot, and don’t get me started on how you join an orcball team.
Which reminds me, if the OP doesn’t mind sci-fi and occasional leftist-bashing there Weber & Ringo’s March books, starting with March Upcountry and March to the Sea. Both are in the Baen Free Library. An immature prince and his regiment of bodyguards are stranded on an extremely hostile planet and have to march across two continents and cross an ocean with only the ammunition and equipment they brought with them.
Seconding Der Trihs. All 4 of the books in the series are excellent, although the last one isn’t so much campaigning as it is political intrigue.
For that matter the Lt Leary series is pretty interesting. I spotted a lot of the old style buy your comission/old boy network of the 17th 18th century wet navies influence.
You can pick up freebies at the 5th imperium website in the wiki.
Nothing at all like what’s been suggested so far, but I found a decent read at my local library the other day: Winter Quarters by Alfred Duggan. Two young Gauls join the Roman army as cavalry auxiliaries and end up accompanying Crassus on his campaign against Parthia. It’s less fantastic than the books already mentioned but religious concerns are often on the main characters’ minds-- specifically, the disfavor of a goddess they’d hoped to have left behind them.
I’m pretty pleased with the book. Part of why it’s so enjoyable is because the last historical novel I picked up on a whim at the library was Wilbur Smith’s Assegai, which didn’t just insult my intelligence, it tried picking a fight.
Say: Draka vs. Dorsai: Who would win? (Assuming rough equality in numbers, armament and technology – including genetic-engineering technology, so, no New Race Homo drakensis unless the Dorsai are similarly enhanced.)
Draka vs. Klingons?
Draka vs. Ewoks?
It was written long before his Lizard invasion series, which I agree is dire.
The Old Man’s War series by Philip Scalzi.
And, of course, The Forever War by Joe Haldeman.
Starship Troopers is worth reading for the characters, politics and military culture; the battle scenes . . . sometimes it’s hard to visualize what’s going on. We never even get more than the vaguest hints of what the Bugs look like.
That would be John Scalzi. And the third book doesn’t have much military action. Also, we’re pretty far afield from medieval/fantasy stories by this point, though the OP did say he was open to other genres.
Yeah, but I saw the movie so I’m all set 
Dave Hartwick, this may sound dumb but thanks for reminding me about my local library. I need to hit their website and look some of these books up and see what they have rather than just buying stuff. Go figure, huh?
Artistic quality always suffers when a writer takes up a cause. But, at least Turtledove did manage to alert millions to the threat of the Lizard People!
How do you feel about a fictionalized account of Vietnam?
Yeah but how far would, I think her actual name was skilly, rather than skully, anyways, how far would she have gotten without the techno ninjas and the arms that senator whats his name provided.
Declan
Just on the military campaign side with zero fantasy element is the Flashman series, which is howlingly funny account of a lecherous, cowardly, racist, greedy, bullying soldier with the British empire.
Another vote for The General series (S.M. Stirling/David Drake) and the Belisarius series (Eric Flint/David Drake), as well as the Empire of Man series (David Weber/John Ringo).
Stirling’s Domination series is short on combat except for the aforementioned Marching Through Georgia.