There’s an old military adage that describes war as long periods of boredom punctuated by moments of sheer terror.

Video-game war is different, because no one would pay $60 for a game in which most of the action consisted of polishing your weapon or playing cards with the guys. So in “Gears of War 3” (Microsoft, for the Xbox 360, $59.99), you’re thrust into one battle after another, with just the occasional chance to catch your breath.

It gets a little boring.

The “Gears” trilogy, which debuted in 2005, tells of the war between mankind and the insectoid Locust on a planet called Sera. At the start of “Gears 3,” the bugs are winning, the last human city has fallen and the Lambent — an even more dangerous mutation of the Locust — are popping to the surface.

All hope rests on the muscle-bound shoulders of Marcus Fenix and his fellow “Gears,” soldiers who once served in the army of the now-collapsed Coalition of Ordered Governments. Early on, we learn that Marcus’ father, the brilliant scientist Adam Fenix, is still alive, and may hold the secret to stopping both the Locust and the Lambent.

And so the Gears begin a long march toward the tower where Adam is being held prisoner. The devastated landscape of Sera is eerily beautiful, but there’s not much time to admire the scenery — hostile bugs lurk around every corner. They range from the compact yet pesky tickers to the appropriately named leviathan, and you never know which mix of monsters is going to pop out of the ground.

You have plenty of weapons to choose from, starting with the series’ signature firearm: the Lancer, a combination of an assault rifle and a chain saw. New toys include the “digger launcher,” which sends explosives burrowing through the ground, and the Silverback mech suit, factory-equipped with machine gun and rocket launcher.

Most of the battles come in one of two flavors: Either you’re ambushed in a tight corridor or fighting through more expansive spaces, which allow for some rudimentary cover and flanking strategies. There’s nothing new here, but developer Epic Games pulls off this kind of mayhem as well as anyone, and the firefights are usually exciting.

The problem is that the single-player campaign settles into a predictable fight-march-fight rhythm. The occasional changes of pace, like a tedious submarine ride, are halfhearted. There are a few flamboyant boss battles, but the only one that really blew me away was at the game’s finale.

Lead writer Karen Traviss does make an effort to turn Marcus and his squadmates into more than relentless killing machines. Dom, who’s mourning his dead wife, and Cole, who’s nostalgic for his days as a star athlete, both get big, dramatic cut scenes, and we’re supposed to grasp the wearying toll that war has taken on Marcus. But these would-be emotional highlights come off as phony, given the constant bro-dude chest-bumping in the action sequences.

The 10-hour story doesn’t quite satisfy, but it does serve as a decent appetizer to a feast of multiplayer modes. The popular (and much imitated) Horde mode, in which players team up to fight waves of Locusts, is back; this time, you can earn money to beef up the defenses around your command post. The new Beast mode flips the script, turning you and your friends into Locusts fighting off waves of Gears.

You can also team up with three other humans and play through the entire campaign. Or you can all turn against each other in the usual competitive modes, like deathmatch and king-of-the-hill. It’s a meaty package that’s sure to remain at the top of Xbox Live’s most-played list for months to come.

As the conclusion of a trilogy, “Gears of War 3” gets the job done, but it’s certainly not the last we’ve seen of this lucrative franchise. I’m hoping that the next trilogy will be a little less predictable — or that the talented designers at Epic will stretch their wings with something entirely new. Three stars out of four.