NEW YORK (AP) _ Try moving a medium-sized city 7,000 miles across an ocean within days to a desert where thousands of people need to eat, sleep, bathe and defend themselves against a million-man army.

That's what the Pentagon is up against in Operation Desert Shield, its largest military operation and most taxing logistical problem since Vietnam.

''You have two things against you - geography and time. And as soon as it gets there, you have to resupply right away,'' said Duane Cassidy, a retired four-star general who until last year commanded the United States Transportation Command, the logistical arm of the military.

''This operation is something no one else in the world can do. Nobody else knows how to do it or practices it,'' said Cassidy, now a vice president for CSX Corp., a transportation company in Richmond, Va.

The Pentagon refused to give specific numbers on troop shipments and which air carriers are involved. But Cassidy estimated 90 percent of the troops and 50 percent of the cargo arriving in the Saudi Arabian desert are being flown on commercial airliners. The Pentagon has used commercial carriers for over 30 years to ferry troops around the globe.

That frees up Air Force planes to carry cargo, such as the C-141, which can hold 200 troops and 34 tons of equipment at the same time, and the C-5, which can hold 340 troops and 130 tons of equipment.

Every 10 minutes, a plane lands in Saudi Arabia. Each non-stop flight takes about 15 hours, and National Guard and Air Force Reserve crews are part of the operation.

Among the U.S. airlines taking part are Eastern, World Airways, Tower International and Federal Express - which gives a military slant to the slogan when it absolutely, positively has to be there overnight.

An all-out sealift is also underway, although the Navy has only eight fast ships capable of carrying tanks, armored personnel carriers and helicopters on the two-week-long voyage to the Persian Gulf. It would take all eight ships just to transport the 24th Mechanized Infantry Division from Savannah, Ga.

The Navy also has about 94 slower ships in a reserve fleet that could be pressed into service.

''The whole United States Transportation Command - air lift and sea lift - is heavily taxed right now. It puts a strain on the system. It's not a crisis, but it's certainly busy and hectic,'' said Capt. George Sillia, an Air Force spokesman at the Pentagon.

Desert Shield is much different from last year's Panama invasion, which was a much smaller operation. Besides, the United States already had forces on the ground in Panama and had its own bases to land its jets.

Desert Shield is also lightning-like compared to the escalation in Vietnam, which by the nature of the conflict occurred much more gradually.

There were 300 U.S. military advisers in Vietnam in 1959. Two helicopter companies were deployed at the end of 1961, raising the total of U.S. personnel in Vietnam to 1,500.

By the end of 1963, the U.S. commitment had grown to 16,500 troops. But the first combat troops didn't arrive until 1965, when a major buildup boosted troop totals to 181,000. Peak involvement reached 550,000 troops. Most of the equipment was sent by sea.

The Air Force, which has the largest fleet of transport planes in the non- communist world, says the Middle East crisis underscores the need to move troops quickly to flashpoints of conflict around the globe.

The Air Force wants to replace its 25-year-old C-141s with the new C-17s. One new transport could do the work of two older planes at less cost, Air Force officers said.

The policy of moving troops and dependents by commercial charter began in President Eisenhower's administration as a way to cut costs. It lessened the need for the Air Force to invest in transport aircraft.

''It's an efficient way to move, cargo and government equipment,'' said Staff Sgt. Mark Johnson of the Military Airlift Command, the long-range air lift arm of the military based at Scott Air Force Base in Illinois.

A 1986 Pentagon study said it would take $2 billion to buy 20 airliners and train crews to fly them. About 95 percent of all Pentagon personnel flying overseas use commercial flights, according to government records.