The further we get away from a war, the more vital it is that there be SOMEbody on a set who can keep the cast and crew from making fools of themselves when you’re making a combat film.

“Military consultant,” “technical advisor,” call them whatever you want. It doesn’t have to be R. Lee Ermey, but you put ex-military on your payroll so that your movie looks and sounds authentic, and not like Spike Lee’s combat movies. Or like Aaron Eckhart in “Ambush.”

It’s a movie set early (ish) in the Vietnam War, 1966. Eckhart plays a commander who sends his Special Forces subordinates to a new base in “Quang TRY province.” Ordering men into harm’s way, and he doesn’t know how to pronounce “Quaug Tri Province?” Like “TREE?”

Maybe it’s early enough in the war that he wouldn’t have heard it. Sure. But considering most everything that follows…

“Ambush” is a B-movie (maybe C) about a lost “secret dossier” that a rageaholic Green Beret (Gregory Sims) and others are sent to recover. Sims goes full R. Lee Ermey “Full Metal Jacket” in the middle of an undersized, remote outpost behind enemy lines, screaming at a subordinate so loud Uncle Ho could hear him in Hanoi.

“What the actual F— does that CHILD need with a gun?”

So, nobody trained the lads in the “This is my rifle, this is my ‘gun'” (penis) rhyme of boot camp?

The answer to that comes when the film’s ostensible star, Jonathan Rhys Myers shows up. He’s a “hunter,” with a tracking dog and a pistol grip pump shotgun. At least the chopper he lands in seems regulation and period correct.

Considering the opening scene is plainly a Jeep-drive through a military aircraft graveyard (a B-58, mixed in with 1980s vintage mothballed jets) meant to be Vietnam era airfield, that’s more than the movie leads us to expect.

“LEFTtenant Col. Mills, sir,” Rhys Myers drawls as he salutes CAPTAIN Mora. Hey, we’re in the jungle. No standing on ceremony. Or British pronunciations of “Lieutenant” from a Southerner.

He’s wearing shiny “Bird Col.” insignia. In the bush, for starters. Shiny target. And he should be wearing the oak cluster of a Lt. Col., right? Entirely too high up the chain of command and too old to be a jungle-savvy LRP (“lurp”). Whatever.

It’s all pretty much downhill from there. The shootouts are noisy and manic but non-military, starting with the “ambush” that opens the action. There’s no rhyme or reason to who we follow and what point of view is dominant.

The party of soldiers sent into the jungle to retrieve the “secret” stuff keeps breaking up into smaller and smaller units — one or two groups sniffing around above ground, another party that splits up when they discover that the Vietnamese dig tunnels and can pop up here, there, seemingly everywhere.

“We’re fah-tin a new enemy,” JRM drawls. “They don’t come from the sky. They don’t come across the sea. They don’t come from the land. They come from the EARTH!”

There’s a lot of tunnel tracking and tunnel fighting, with firearms, knives and a flamethrower. The booby traps show SOMEbody Googled “Vietnam War” to find “punji pits,” and a cute scorpion dump.

It’s not good. I can’t vouch for any of the three credited screenwriters so much as rented “Platoon” on video as the most basic “homework.” Seeing as how much of the audience for such films is current and ex-military and genre buffs, that’s not smart, kids.

I dare say most of the cast is better than this sort of material, with one or two RANTING exceptions.

But whatever blowback you fling at your agents, guys, memorize this. Don’t sign on to a combat film without a military technical consultant on set from the start of rehearsals. Make sure there are rehearsals. And “This is my rifle, this is my gun…”

Rating: R, graphic violence, profanity

Cast: Jonathan Rhys Myers, Conor Paolo, Patrick Walker, Jaime López, Mac Brandt, Gregory Sims, Frances Mancho and Aaron Eckhart

Credits: Directed by Mark Burman, scripted by Mark Burman, Johnny Lozano and Michael McLung. A Saban release.

Running time: 1:44