WITH U.S. FORCES, SAUDI ARABIA, NOV. 24 -- The U.S. military has spent much of the past decade preparing to fight a low-intensity war in jungles or forests. Now it suddenly faces the prospect of tank warfare in the desert, and commanders are scrambling to reorient troops and equipment for this vast, hostile terrain and for the largest, most heavily armored battle front since World War II.

Commanders acknowledge that they have been forced to retrain troops more accustomed to tropical forests and tree-covered hills than the flat, open desert where advancing tanks send columns of dust high into the sky, a convenient warning to enemy forces.

Combat troops with limited experience in nighttime maneuvers are carrying out a large portion of their operations in the dark in an effort to confuse enemy forces and hone skills for possible surprise night attacks.

The American forces have gotten so lost, so often in the featureless desert that they are rushing new, computerized ground-to-satellite navigation aids to troops on the Arabian peninsula.

"When we first came out here we were overwhelmed by how austere it was and how difficult it would be to fight," said Army Capt. Mike Garner, a member of the 82nd Airborne Division, a prototype of the military's new mobile, lightly armed units. "We're trained for a low-intensity scenario; here it's mechanized warfare in the wide-open desert. Initially, it was a tough transition. . . .It takes a lot of work just to get soldiers to the point that they learn to survive."

Moreover, U.S. forces trying to sharpen defensive combat skills for their stated mission of protecting Saudi Arabia against an Iraqi invasion now are working on more offensive tactics to prepare for a possible attack against Iraqi President Saddam Hussein's forces.

Although military commanders here say they have been satisfied with the progress of their training operations over the past three months, they readily concede that they could use even more preparation and admit relief that they didn't face combat in their first days here.

"Anybody would be a fool to say we wouldn't want another few days," said Col. Carl Fulford, commander of the 7th Marine Regiment, which trains in the desert at its home base at Twentynine Palms, Calif. "But we are a hell of a lot more ready now" than when the troops arrived.

"It's always better if you have time to prepare," said Brig. Gen. Steven L. Arnold, the assistant Army chief of staff for the Middle East. "But the more time you have to prepare, the more time the enemy has."

Despite the critical need for artillery and tank crews to practice firing their weapons in an environment that can alter the performance of their equipment, some training is hampered by Saudi restrictions on live-fire exercises and U.S. concerns that limited ammunition be conserved. Military officials, concerned that some combat troops could lose their fighting edge sitting in the desert without adequate training, have ordered high-technology weapon simulators that allow troops to practice their skills using small, automated reproductions of tanks, laser guns and other equipment. To conserve ammunition and avoid littering the Saudi desert with unexploded shells, many military units have ordered dummy ammunition from the United States and Europe.

Already, the Saudi desert operation has revealed shortcomings in the military's recent efforts to pare equipment and budgets in response to anticipated changes in its future battlefields.

When the Marine Corps opted for lighter, more lethal weapons several years ago, it decided to junk its huge, 25-year-old self-propelled howitzers. The cannons, mounted on chassis much like those of tanks, can rumble across varied terrain on their own tracks, stop and hurl 204-pound projectiles toward the horizon. Although the guns were removed from all Marine units, they remained on prepositioned ships -- vessels packed with arms and supplies and placed at bases around the world for quick deployment -- which were dispatched to Saudi Arabia soon after Iraq invaded Kuwait on Aug. 2.

Marine artillery units hastily retrained some soldiers to operate the 8-inch cannons and sent them into the desert. While trucks towing newer, non-motorized howitzers have frequently bogged down in the sand, the older models have plowed through the dunes on their own.

"We're certainly glad they were still on the ships," said Lt. Col. Jim Sachtleben, who commands one of the Marine artillery units here. "Saddam Hussein saved that howitzer."

Even units in the first wave of 240,000 troops dispatched to Saudi Arabia that had trained in U.S. deserts and were assigned to Middle East contingency forces have required additional training to adapt to the vastness of the Saudi desert.

Nothing has disrupted U.S. operations more than the difficulty of navigating a vast sea of shifting sands that can obliterate a road that was visible two days earlier, where the horizon is not where it appears to be and where there are few distinctive landmarks to guide troop movements. Tank crews, infantrymen and helicopter pilots have spent hours lost in the desert.

To cope with the problem, the Pentagon has ordered hundreds of computerized navigational aids to be sent to the Saudi desert. Much of the equipment uses the Global Positioning Satellite system, which can track coordinates anywhere on earth. But some military commanders say the satellite is sometimes difficult to pick up because of its position in the sky, particularly in the middle of the day.

Forces accustomed to staging attacks and counterattacks from behind thick forests and steep hills are now revising their tactics to contend with seemingly endless stretches of barren terrain that provides few obvious hiding places. Because the visible distances are so great, soldiers have had to learn to pace their attacks to be certain their targets are within range of their weapons.

Because the long distances make it difficult to identify troops and equipment, the Army has found that adequate communications can mean the difference between destroying an enemy and wiping out an ally.

Military officials have said communications in the desert are hampered not only by rapidly changing atmospheric conditions but by the mineral content of the sand, which can interfere with some radio frequencies.

While the military has slowly moved toward a greater emphasis on nighttime operations since the Vietnam War, most pilots, ground troops and other forces here have been required to increase radically their night missions to prepare quickly for possible combat against Iraqi forces.

"In Vietnam we never conquered the night. It belonged to the enemy," said the Marines' Fulford. Now, contends the Army's Arnold, "The night really belongs to us."

The military's ability to operate at night has been helped greatly by new, high-technology night vision equipment for air and ground forces. The pilots' goggles contain 1.5 million microscopic tubes that magnify starlight and other minute light sources to give the murky black desert a glow that resembles daylight. Helicopters invisible to the naked eye loom clearly when scanned with the goggles; tiny Bedouin campfires on the desert floor stand out like spotlights, and unseen military encampments appear bathed in greenish light.

The goggles, however, have potentially deadly problems. Bright lights can temporarily blind a pilot wearing the goggles. The glasses also tend to distort distances and are virtually useless in clouds of sand.

One recent Marine exercise, intended to simulate flying troops to an enemy command center and airlifting them out of the area after they have destroyed it, highlighted the difficulties of night operations.

Pilots initially were unable to spot their landing zone, a dark patch of hard sand. Through the goggles, they mistook it for a shadow.

The pilots were then plagued by a bright gas-pipeline blaze that "shut down" the goggles, obscuring the tiny signal lights arranged on the ground to guide them to their landing zone. When the pilots regained their vision, some complained that signal lights could not be distinguished from soldiers' flashlights.

Lt. Col. T.J. Frerker, the squadron's commander, said the Marines require their helicopter pilots to complete at least 25 hours of training with the goggles after they arrive in Saudi Arabia. "We need to practice some more," he said. "There's a lot more to these things than just putting them on your face and going out flying."

Helicopter pilots called off one recent Army night-flying exercise when, wearing the goggles, they lost track of their positions and commanders feared they would collide, according to one participant.

Five U.S. military helicopters crashed at night during the first two months of operations in Saudi Arabia, but at least one crash occurred after a flight crew had removed its goggles, according to an Army offical. News reports have linked various versions of the goggles to at least 183 deaths in 88 military helicopter crashes since 1978. The Pentagon has blamed such crashes on pilot error, according to those reports.

Ground forces also have begun to refocus much of their training on night maneuvers. Few units now change their positions, even for training exercises, without the cover of darkness. Troops are practicing even routine food and fuel resupply missions at night, something rarely done before the Saudi operation, according to Fulford.

Fulford said parts of his regiment waged a simulated 15-mile attack on a recent moonless night. "It wasn't perfect, but it was better than it would have been 10 or 15 years ago," Fulford said.