The U.S. military is constructing a vast logistical system in the Persian Gulf that will be capable of sustaining more than 400,000 troops for at least a year if President Bush decides to delay going to war against Iraq, according to senior American planners.

Keeping such a huge force poised for combat until next winter would be expensive and difficult, but clearly manageable, the planners said.

To feed, clothe, shelter and arm the troops, the staff working for Maj. Gen. William G. "Gus" Pagonis, chief logistician for Operation Desert Shield, has grown from 300 in early August to 25,000 now. The logistical infrastructure being built will be serving more than 1 million meals and delivering as much as 700 tons of mail every day.

A fleet of more than 2,000 trucks has been assembled to distribute the nearly 80,000 tons of materiel the Defense Department estimates will be needed every month to sustain the force -- a cargo roughly equivalent to hauling the Washington Monument across the desert, stone by stone.

Gen. Colin L. Powell, chairman of the Joint Chiefs of Staff, acknowledged in congressional testimony last week that the 430,000 U.S. troops committed to the gulf could be maintained for a year, even without fully activating the reserves. "It is a daunting challenge which is taxing us," Powell said, "but it is a challenge which is very, very manageable in my judgment."

Since Bush announced the virtual doubling of U.S. forces in the gulf last month, some congressional leaders and military analysts have questioned whether the deployment could be sustained for more than a few months. Their worry has been that Bush would be forced to "use or lose" the force because of overwhelming long-term logistical problems -- a concern raised repeatedly by, among others, Sen. Sam Nunn (D-Ga.), chairman of the Senate Armed Services Committee. "If you double the number of forces over there, you're going to have a very hard time having enough sustaining power to be able to maintain those forces, even without a war," Nunn warned in mid-November.

Bush himself has referred to "a ticking of the clock" limiting the time left for a peaceful resolution of the gulf crisis.

But military planners in the United States and Saudi Arabia now believe that improved supply lines and depots, increased Saudi assistance and the experience gained so far from Operation Desert Shield mean the deployment can be sustained at least through next year. Officials also hope to avoid some of the snafus that plagued the initial deployment by substantially beefing up logistical support in the gulf before the new wave of troops arrives. If the clock is ticking, they suggest, it should not be because of logistical concerns.

At the same time, keeping 430,000 troops in the Mideast will exact an enormous price, even if the logistical problems are mastered. Without a shot being fired, Desert Shield will cost about $30 billion in fiscal 1991, administration sources estimate, and the harsh desert environment will take a toll on a broad variety of equipment and weapons, from tents to tanks.

Moreover, because most Marine and heavy Army combat forces are already deployed to the gulf, there are not enough fresh troops to replace those in the desert. Commanders fear that lack of rotation will cause severe morale problems if the crisis drags on beyond this winter, a consideration that could weigh as heavily as the issue of sustainment. To keep the force at full strength, the military's recently announced "stop-loss" policy of sharply limiting retirements and resignations may have to go on indefinitely.

"I think from a pure military standpoint we could sustain them for a long period of time and we could keep the troops over there for years just like in World War II," retired Gen. David C. Jones, former chairman of the Joint Chiefs, said recently. "Not that it wouldn't be at a great cost."

Whether the nation has the political will to sustain the force for long is an issue over which military commanders say they have no control; instead, they are focusing on the physical factors needed to support the huge U.S. presence in the gulf.

To accommodate the additional 200,000 troops and their equipment, military construction units are expanding aircraft ramps and parking aprons, building maintenance hangars at airfields and ports and laying roads across the desert. Traditionally, the "tooth-to-tail" ratio of combat troops to support troops has been roughly 1 to 3; for Desert Shield, a Pentagon spokesman said yesterday, the ratio could grow to 1 to 5 because of the distance and duration of the operation.

Port capacity has been doubled in recent weeks, according to Desert Shield logistician Pagonis, who said his 2,000 trucks each make an average, one-way haul of 230 miles across the eastern province of Saudi Arabia every day.

Perhaps the most critical supply need, except for food and water, has been munitions. Although huge quantities of bombs, artillery shells and ammunition already were repositioned in the gulf region, military planners found certain stocks -- particularly high-tech missiles -- in short supply. To fill the void, the services are moving substantial numbers from Europe and the United States, while also ordering new production runs.

By last month, for example, the Army had tapped its European stocks for 1,000 Hellfire and 3,000 TOW II anti-armor missiles, 4,000 105mm artillery rounds and 900,000 rounds for 25mm cannon, according to senior Army officials. The Air Force in the first weeks of Desert Shield persuaded Congress to provide an extra $40 million to buy about 600 GBU-27 laser-guided bombs, which Texas Instruments is making.

In general, Pentagon officials said, munition stockpiles are nearly full except for a few categories of war reserves; to avoid dipping excessively into those war stocks, commanders in Saudi Arabia said, "live fire" training sessions have been limited.

To bolster the small army of U.S. logisticians working on Desert Shield, the military has turned to local vendors, contracting for billions of dollars worth of truck rentals, food services and equipment. Because of the urgent need to supply the troops, the military initially bypassed normal bidding procedures to purchase items as diverse as rice, Bedouin-style tents and lumber used to build latrines.

Some Saudi merchants began price-gouging, according to several U.S. military officials. For example, the military's Joint Information Bureau in Dhahran was charged $1.3 million to lease four buses for a month, a military spokesman said.

"The GAO {General Accounting Office} is going to come in and have a field day when this is over," said one official. "They'll find accounting problems, excessive pricing, all kinds of things -- but it's all items we needed in a hurry and didn't have the luxury of shopping for the best prices."

To exert greater control over vendors, the Saudi government agreed to reimburse the U.S. military for all services and equipment purchased after Oct. 31, according to U.S. officials. Even before Nov. 1, however, the Saudi government had provided hundreds of millions of dollars worth of services and goods to the U.S. military, including about 90 percent of all fresh food consumed by U.S. troops.

The Saudis recently turned over a $760 million check to the U.S. military to cover some of those costs. A military officer flew to New York aboard the Concorde carrying the check for deposit. Officials said the check was rushed to the bank because it would earn about $1 million a day in interest.

The confidence military officials feel in their ability to sustain the force for at least a year stems in part from the generally good performance of U.S. equipment -- and mechanics -- in the desert environment.

There have been problems, to be sure. The Air Force has had to replace more windshields than usual on low-flying jets such as the F-111 and F-15E because blowing sand pits the plastic badly; the Army and Marine Corps have been forced to wrap helicopter blades in adhesive tape to slow the corrosive effects of sand; the time between overhauls of some Chinook helicopter engines has been cut from an average of 300 or more flying hours to about 50 because of dust; and the combination of more sorties and fewer maintenance opportunities means that the asphalt-like coating on some aircraft carrier decks is quickly wearing thin.

On the other hand, the desert capabilities of many tanks and armored personnel carriers have exceeded expectations. Not a single Air Force jet engine has been pulled because of excessive grit deposits from the calcium-rich sand on the Arabian Peninsula, according to a senior Air Force official. Although the C-5 and aging C-141 jets used to haul troops and cargo to the gulf are flying four to five times as much as before Desert Shield began, the "structural life" of the aircraft actually has been prolonged since the missions impose relatively little stress on the planes and they are no longer used to train inexperienced new pilots. And A-10 attack planes often work better in Saudi Arabia than at home, where humidity takes a toll.

Keeping the U.S. jets, tanks and ships fit to fight for another year will require an extraordinary maintenance effort, U.S. logisticians said. Many mechanics now work 12 hours a day, six days a week -- a commitment spawned in part by the dearth of other diversions in the gulf. Maintaining sufficient spare parts and rested crews for transport aircraft also is expected to be a difficult, though not impossible, task.

In the immediate future, the dual tasks of supporting the first wave of troops and moving in the second deployment is expected to tax military transportation systems more than originally anticipated, officials said. The Military Sealift Command has nearly doubled the ships used for Desert Shield, from 138 vessels at the height of the initial deployment to 249 now.

In the last two weeks, 60,000 tons of food, clothing, spare parts and sundry other items has been shipped to Saudi Arabia. In four months, the military and contracted civilian airliners have flown 6,900 missions and 215,000 passengers to the Saudi peninsula, an intensity that will continue well into January.

Beyond that, planners are hedging their bets by simultaneously preparing for war and a peaceful return to the United States. For the latter, Pagonis said he has assembled a "small planning cell" and has also ordered 1,000 steam cleaners to scrub "our gear so we can pass the U.S. agricultural inspection and get stuff back into the U.S."

Staff writers Guy Gugliotta in Saudi Arabia and George C. Wilson in Washington, and staff researcher Lucy Shackelford in Washington, contributed to this report.