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 U.S. 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 U.S. 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 U.S. territory, says he believes that 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 U.S. military official acknowledged is fraught with difficulties.

The buildup plan, to be carried out by 2014, represents a major realignment of U.S. forces in the Pacific. It is designed in large part to ease the long-standing overconcentration of forces on Okinawa, the U.S. military's key Pacific outpost since the 1950s, without pulling them back too far from such potential flash points as Taiwan and North Korea.

But keeping the Marines at their present levels on Okinawa has become unrealistic.

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

  Okinawans have long complained that their crowded island has to absorb too much of the U.S. presence, with the attendant crime and misbehavior of military personnel stationed there. More than half of the U.S. troops in Japan are on Okinawa, which is home to Kadena, the biggest U.S. air base in the region.

In U.S.-Japanese negotiations, Guam emerged as the most practical alternative.

Okinawans have generally welcomed the move, and Tokyo has pledged to invest nearly $3 billion in building barracks, offices and other facilities for the troops on Guam, and to lend another $3.3 billion for developing supporting infrastructure.

Roughly 10,000 Marines are to stay on Okinawa, however, and Tokyo has run into serious opposition in trying to move the Futenma Marine Corps Air Station to a less congested part of Okinawa. Many Okinawans want it off the island altogether.

Lt. Gen. Edward Rice, commander of the U.S. forces in Japan, says the whole move to Guam depends on Futenma getting new premises on Okinawa.

“;There are serious and significant challenges that remain for us to facilitate the transfer,”; he said in Tokyo.