WITH U.S. FORCES, SAUDI ARABIA, AUG. 23 -- The sun was searing the desert floor at 110 degrees today when a Marine rifle company was ordered into full chemical-warfare combat gear. They struggled, panting and sweating, into thick coveralls, black rubber boots and masks that left them looking like a troupe of Darth Vaders.

Twenty minutes into the practice drill, one Marine went down, collapsing in a green heap on the sand. "Get his jacket off -- cool him down!" shouted one of his colleagues, as others ripped off his mask and began pouring bottles of water over his head.

In Saudi Arabia, heat has become the most hostile enemy of U.S. troops. Even more than the nearly 200,000 Iraqi soldiers massed across the Saudi border in Kuwait, the brutality of the sun has been the most dominant force in shaping the initial operations of American ground forces here.

It has prompted military commanders to slow planned operations and has radically disrupted training routines. It is reshaping the lives of even the most hardened Marines and Army troops, exacerbating already austere and stressful conditions.

"The hardest thing we're dealing with here is the combination of the heat, the hours, the sand and the dust," said Capt. Adrien Burke, 29, commander of the Marine landing-support detachment that is directing the flow of tons of Marine equipment being flown into Saudi Arabia. "It makes for long, dusty days and short, sleepless, uncomfortable nights."

The 20 men of Burke's unit have been camped under a dust-covered tent on the edge of the airfield, without showers or hot meals, since they arrived 13 days ago. They dine on military-issue Meals Ready to Eat -- from beef stew and scalloped potatoes to ham cubes and chicken a la king.

"The biggest break of the day is to sit in the shade and drink water," said Burke, who, like every American commander on the ground in Saudi Arabia, orders his men to drink at least 4 to 6 gallons of water a day.

In the first days of Operation Desert Shield, many U.S. troops have spent as much time acclimating to the harshness of the desert as they have establishing defenses for Saudi Arabia. "You never get climatized," grumbled a soldier from the Army's 82nd Airborne Division. "You just learn to tolerate it."

A Marine sentry guarding a group of tanks today sought a fraction of relief by standing in the shadow of a skinny light pole. American troops have arrived during what is described as the cruelest month in this region -- the time of year when Iran and Iraq ceased most combat during their eight-year war because of the toll the climate took on both men and machines.

"When they opened the doors of the plane the first night we got here, I nearly had a heart attack -- it was 114 degrees," said a 29-year-old combat signal specialist from Fort Bragg, N.C.

Many of the troops, who normally wear lightweight green jungle fatigues on exercises, say they have been dismayed to find the new sand-colored desert fatigues are as heavy as winter uniforms -- so they can stand up to the rocks and rugged desert terrain that would shred thinner fabric.

Commanders of the 7th Marine Expeditionary Brigade, arriving from three California bases, are rotating troops into the desert on four-day training missions in preparation for extended deployments among the monochromatic desert dunes.

Twentynine Palms, a tough training ground in California's Mojave Desert, "is a lush vacationland compared to this place," said a Marine on exercises in the Saudi desert Wednesday.

After a few days of operations under the Saudi sun, a growing number of commanders, particularly those leading units in the shelterless desert, have ordered their troops on "reverse cycle" -- sleeping by day, training during the relative coolness of the night.

Troops have discovered, however, that the night can also be treacherous, as poisonous black cobra snakes, giant scorpions and mosquitoes emerge from daytime hideouts.

As the furnace-like winds and blistering heat keep medical teams working full-time to combat heat stress, rashes and dehydration, the brutal desert is taking a similar toll on military equipment.

Marines guarding a port entrance tape bits of cloth over their M-16 rifle barrels to keep out dust and grit. "Our weapons are so precision-made, the sand is hard on them," said one Marine officer, who noted that the sand may affect the reliability of some arms. The Saudi sand, which seeps into the tiniest crevices, is so powder-fine that it can't even be used to make concrete, and the Saudis must import coarser sand for construction.

The heat and sand also are exacerbating the overcrowded, austere living conditions of troops in transit to their eventual assignments. Tent cities have sprouted at airstrips, ports and staging sites in many sections of the country. The Saudis also have provided dozens of makeshift bunkhouses and offices.

Marine helicopter pilots and maintenance personnel have taken over a new Saudi military airport that had not been used before this deployment. Sleeping bags are strewn across floors, and maintenance shops have been created in every room and spare corner. But it has running water and air conditioning, and by comparison with rough quarters elsewhere is a plush accomodation. For example, at a disembarkation port where thousands of tons of military supplies and hardware are being unloaded, hundreds of Marines are bunked cheek by jowl in airless tin warehouses.

"The Saudis actually apologized to us for not being able to put us up in hotels," said one incredulous soldier. The Saudis have begun putting up air-conditioned, prefabricated barracks for American forces in some areas.

The spartan conditions have prompted unusual camaraderie among traditionally competing services. The Air Force, which is housing some of its personnel in an air-conditioned building, has opened its showers to busloads of Army soldiers camped in tents on the fringes of the airfield.

The military is shipping thousands of air-conditioning units to the region to cool hospital tents, command posts and buildings and tents used to house sensitive computer equipment.

As for diversions, troops on exercises in the desert wave to passing nomads. The Marine rifle company awoke after its first night camped in the desert to find a herd of camels peering at them curiously from the top of a nearby sand dune.

"They all raced for their cameras," said an officer.