One of the bloodiest and most important struggles of World War II in the Pacific is given a loopy, ahistorical B-movie treatment with loopy, B-movie actors in “Battle for Saipan.“

It’s loosely based on the real-life “banzai charge” at the climax of the battle, when thousands of surviving Japanese troops charged vainly at Marine and Army positions, overrunning them at first but wiped-out, almost to the last man, eventually.

Writer-director Brandon Slagle (“Frost,””Crossbreed”) uses that fact for a fictional “last stand” story set in a military hospital, with doctors and nurses having to pick up weapons to fend off bayonets, machine guns and samurai swords.

Setting it in a hospital also allows Slagle to make his WWII movie coed, so that he could cast his muse — Devanny Pinn — something he does in every film of his I cared to look up. Isn’t that sweet?

The movie, filmed in Thailand, has Louis Mandylor as a major whose patrol is ambushed at the beginning of that July 7 Japanese counter-attack. Major Porter is from Staten Island, a fact that’s an endless bone of comical contention when he drags the one other survivor from his unit to the hospital, where Dr. Vic (Casper Van Dien) presides.

Vic is from Brooklyn. Staten Island might as well a Canadian province.

Porter is taken to “the general,” played by Jeff Fahey in full Col. Kurtz/”Apocalypse Now” mode. He’s sweaty, goateed, long-haired and drunk. And he’s supposedly in charge. Guess it’s up to the major to organize a defense.

“Do you hide beyond your scalpel and syringe of morphine all day?”

Everybody in the place, doctors, nurses to patients — including the guy we see lose his arm (graphically) in a surgery scene — has to fight for their lives as freshly-laundered Japanese infantry and officers infiltrate the place, generally in groups of two or three, making them easier to handle.

There’s no urgency to the situation. A Cold War era tank meant to be Japanese blows a hole in the building, and all the ward of patients and nurses can do is shrug and ask “What the hell was that?”

The fights are reasonably well-staged, with the odd ludicrous touch.

But this is weak directing of bad writing with a lot of actors — one of them apparently the director’s lady friend — getting a working vacation in Thailand for their trouble.

There are epic tales about this bloody battle waiting to be told. One, “Windtalkers,” was turned into a movie. Making this one up seems all the more pointless seeing as how poorly it turned out.

Rating: R for strong violence, bloody images and some language

Cast: Casper Van Dien, Louis Mandylor, Devanny Pinn, Jennifer Wenger, Natalia Nikolaeva, Randall J. Bacon and Jeff Fahey.

Credits: Scripted and directed by Brandon Slagle. A Saban Films release.

Running time: 1:33