“Is it terrorists?” terrified teenager Robbie (Justin Chatwin) asks his dad Ray Ferrier (Tom Cruise) at the outset of the alien invasion that drives Steven Spielberg’s War of the Worlds--and you can understand his confusion. Characters find themselves covered in dust that comes from the ashes of vaporized bodies; survivors stumble past a wall plastered with flyers announcing missing family members; the whine of a jet engine announces destruction. Though ostensibly working from the 1898 H.G. Wells story that inspired a legendary radio broadcast and a previous 1953 film version, Spielberg sometimes seems to be making the first, risky attempt at September 11: The Motion Picture.

Yet he’s also making an old-fashioned disaster movie, the kind of “event” thriller that no other filmmaker has ever crafted with more rousing, crowd-pleasing skill. War of the Worlds sometimes finds the director working at the top of his game, and sometimes finds him struggling not to fall back on easy tricks while dealing with such heavy subtext. It’s the Spielberg of Schindler’s List arm-wrestling the Spielberg of Jurassic Park for control of the material, resulting in a film both nerve-jangling and frustratingly generic.

For most of the film’s first half, it’s squarely in the nerve-jangling category. There’s an eerie quiet--rarely unbroken by John Williams’ music--to the early scenes of divorced dad Ray and his kids Robbie and Rachel (Dakota Fanning) finding their weekend together disrupted by freakish occurrences. Lightning strikes, electricity and cars fail, and soon massive mechanical tripods are rising from beneath the earth to fry unsuspecting onlookers. From the initial assault through the Ferriers’ grim quest for anywhere safe, War of the Worlds often works as well at being simply unsettling as it does at being big and momentous--M. Night Shyamalan mixed with Roland Emmerich.

And as the film dives into the dehumanizing fallout of a world under attack, Spielberg finds moments of bleak power. A column of human survivors at a railroad crossing sees a train hurtling past them with flames streaming out its windows like commuter rail to hell; one of the few functioning automobiles inspires a homicidal mob. Even our hero is forced to contemplate murder when Ray and another survivor (Tim Robbins) clash over how to deal with their situation. As post-apocalyptic, age-of-Homeland Security scenarios go, this one shows a streak of daring misanthropy.

Then there’s the part where Spielberg realizes hundreds of millions of dollars are on the line. First there’s the presence of Tom Cruise, whose character is a divorced New Jersey dock worker--and if you guessed that some frustration comes from “divorced” being the only part of that description where Cruise feels believable, you’d be right. While Cruise is fine as a concerned parent--he’s more convincing caring for children than he is dating them--his movie-star presence is a constant reminder that this is a big blockbuster rather than the dark character piece it sometimes pretends to be. Screenwriters Josh Friedman and David Koepp turn Wells’ story into the same narrative that his driven disaster movies from Earthquake to The Day After Tomorrow: loved ones divided. It’s hard to work up genuine enthusiasm for the fiftieth variation on that theme.

Spielberg himself sometimes seems to be having the same problem. It’s not entirely fair to look at a movie this consistently tense and accuse him of phoning it in, but some of his choices here feel like the work of someone who opts for the familiar when he’s not sure what to do. That’s most evident during a sequence in which Cruise, Fanning and Robbins hide out silently from probing mechanical tentacles--a bit nearly identical to one in the last Spielberg/Cruise collaboration, Minority Report. That it still works as well as it does says more about Spielberg’s mastery of his craft than it does about how much his heart is in the set pieces.

In the end, when this War of the Worlds finds the same weakness in the alien menace that Wells did over 100 years ago, little inconsistencies start to nag. They’re the kind of things that wouldn’t necessarily bother you in a fully enthralling movie, but here serve as reminders that there was a constant tug-of-war between serious post-9/11 allegory and happy-ending Hollywood spectacle. The never-named invaders of War of the Worlds aren’t terrorists, but this solid piece of genre entertainment is most effective when they sort of are.

WAR OF THE WORLDS *** Starring Tom Cruise, Dakota Fanning, Tim Robbins. Directed by Steven Spielberg.