If the teenage girls sitting in the preview screening were giggling at this film's awkward sentimentality and its nosedive of an ending, maybe a rewrite was in order. The latest tear-jerker to be adapted from a novel by Nicholas Sparks ("The Notebook," "Nights in Rodanthe") has a compelling premise but stumbles often, especially in crunch time.

"Dear John" is meant to be a star-making role for Channing Tatum, who plays a U.S. Army Special Forces soldier named John Tyree. Starting in the spring of 2001, we follow him off to various war zones as he uses handwritten letters (they didn't have e-mail back then) to stay connected to Savannah, the blond, green-eyed love of his life.

The movie takes place over seven years, starting with the couple's star-crossed romance on the beaches of South Carolina. Savannah (Amanda Seyfried) is a big-hearted rich kid with snobby parents. John is a former ruffian raised by an autistic father (the great Richard Jenkins, an Oscar nominee for 2008's "The Visitor"). Their attraction is instant. "Two weeks together, that's all it took," Savannah says.

Cue acoustic love song.

One problem: The Sept. 11 terrorist attacks, which send a patriotic tidal wave through the ranks, carrying John off to re-enlistment. "Tell me everything," Savannah says. A rousing montage of letter writing ensues, cutting back and forth between Savannah daydreaming in class and John dodging Taliban bullets.

If we've learned anything from Sparks adaptations, it's to be prepared for a barrage of sappy plot twists. One, concerning John's dad, actually contains some welcome earnestness. But it's quickly brushed aside for more Sparksian schmaltz.

So what about the hulking, hypermasculine Tatum? In "Dear John," he once again plays the alpha brute -- his physique sculpted like an action figure, or maybe an underwear model (he's been both). While he's a perfectly acceptable young actor, Tatum sometimes coasts through scenes as if he'd rather be someplace else. Indeed, his career reads like a mathematical equation for turning beefcake into bonafide stardom. He's covered his bases with a teeny-bopper hit ("Step Up"), a gritty indie flick ("Battle in Seattle"), an action blockbuster ("G.I. Joe") and now a leading-man drama. Could a romantic comedy with Rachel McAdams be next?

But enough about young Tatum. It was director Lasse Hallström's job to keep "Dear John" on course. The Swedish filmmaker had a great run at the turn of the century with Oscar-nominated films "The Cider House Rules" and "Chocolat." Since then, he seems to have lost his luster.

In "Dear John," Hallström has a serious and timely premise: How do relationships survive the separation caused by modern war? Instead of giving us sincerity -- and the dewy-eyed romantic fireworks this movie calls for -- we're left with a giggle-worthy mess.