ANDERSEN AIR FORCE BASE, Guam - Sprawling toward the horizon in every direction, Andersen Air Force Base is surprisingly quiet, leaving the impression of a big, empty parking lot. For now, anyway.

Over the next six years, nearly 25,000 US Marines, soldiers, family members, and civilian Defense Department employees are to descend on the tiny Pacific island of Guam, transforming the sleepy tropical outpost into a hub of America's military in the Pacific.

But the metamorphosis seems as fragile as it is ambitious.

Guam's transformation will cost at least $15 billion - with Japan footing more than $6 billion of the bill - and put some of the US military's highest-profile assets within the fences of a vastly improved network of bases. The newcomers will find an island already peppered with strip malls, fast-food franchises, and high-rise hotels serving Japanese tourists who want a closer-to-home version of Hawaii. The plans for the base are fueling a fresh construction and real estate boom which Guam hopes will accelerate its prosperity.

But Guam is smaller than some Hawaiian islands, with a population of just 155,000, and many of its officials are worried that the military influx could leave the island's infrastructure - water, highways, and seaport - overwhelmed and underfunded.

Felix Camacho, the elected Republican governor of the US territory, says he believes in the long run the troop influx will be "tremendous" for Guam's economy, but it will be "a difficult and complex process."

"I remain hopeful," he said in an interview. "Our challenge is that we know that the Department of Defense and Japan will build a first-rate base." But Guam has "limited capacity" to develop its own infrastructure to absorb the influx, he said.

Joe Murphy, in a recent editorial in the Pacific Daily News, Guam's main newspaper, focused on the upside. "The shift of Marines may cause problems," he wrote, but "Transportation should get better. Our nightclubs should get better. So should our restaurants and movie theaters. It all should trigger an advancement in the social scene on Guam. This is a new era, and we've got to move forward."

However, the whole plan could collapse if Japan fails to build a replacement for a busy Marine Corps air base on its southern island of Okinawa, a festering issue that one senior US military official acknowledged is fraught with difficulties. The buildup plan, to be carried out by 2014, represents a major realignment of US forces in the Pacific:

About 8,000 Marines are to be shifted 1,200 miles southeast, from Okinawa to Guam, making it the Corps' second largest permanent overseas staging and training area.

The Navy has already deployed three nuclear-powered submarines to Guam and is seeking improvements to accommodate the nuclear-powered aircraft carrier USS George Washington, which carries about 5,000 sailors and airmen.

The Army wants to deploy a ballistic missile defense task force, which would bring roughly 630 soldiers and 1,000 dependents to Guam.

Long-range B-2 bombers have begun regularly deploying to Guam, along with squadrons of F-16 fighters. Military planners are considering bringing in the new F-22 fighters as well - though details remain sketchy - along with Global Hawk unmanned surveillance aircraft and a dozen tankers.

The buildup is designed in large part to ease the longstanding overconcentration of forces on Okinawa, the US military's key Pacific outpost since the 1950s, without pulling them back too far from such potential flash points as Taiwan and North Korea.

By treaty with Tokyo, more than 50,000 US troops are stationed throughout Japan, which pays billions of dollars each year to support them, more than any other country with a US base on its territory.