WITH U.S. FORCES, SAUDI ARABIA, SEPT. 5 -- Under a brilliant full moon, India Company is digging out the sandy foxholes that will be the front-line ground defense for American forces if Iraqi troops and tanks roar across the border of Kuwait into Saudi Arabia.

Each night, hundreds of U.S. Marines troop into the desert from their daytime hideaways in dusty limestone quarries to build the network of sandbag bunkers that will serve as small fortresses for the infantrymen protecting critical ports and oil refineries here.

"We will be the first ones to engage the enemy if we are attacked," said Maj. Michael Vrabel, executive officer of the Marine Corps battalion assigned to defend one sector of Saudi Arabia's Eastern Province, which abuts Kuwait.

It is that mission that gives the Marines here a sense of urgency and intensity unmatched among U.S. troops in Saudi Arabia. They labor by night, not only to escape the blistering rays of the desert sun but, more importantly, to help avoid detection by Iraqi forces, according to commanding officer Lt. Col. Larry Wright.

This battalion's base camp is a yawning hole in the desert floor where Saudis mine limestone to feed nearby factories. The mounds of rubble and sand that ring the quarry provide a natural fortification for the 1,000 men who live inside under camouflage netting coated with chalky white lime dust.

By day, the base is a vast sunbaked crater dubbed "Camp Griddle" by some of its Marine residents. By night, it is a surreal moonscape of rocks and ridges. Commanders said, however, that the quarry is an ideal desert hide-out, shielding troops from enemy view and giving them the advantage of launching a surprise "pop-up" attack against advancing forces.

Under one tent is the "war room," where commanders have built a large grid map in the sand, depicting their positions and the potential avenues of approach that could be used by Iraqi forces. As soon as troops have finished building their foxhole bunkers, commanders will begin conducting emergency drills, scattering the infantrymen to their desert posts. The exercises will be repeated night after night, with constantly changing scenarios.

"The desert is a fast-moving combat environment," said Capt. Jeff Sherman. "You have to make split-second decisions. There's no time to pause and think it over."

Miles of thin black telephone wires snake across the desert floor, linking the command-and-control tent with units in the field. They replace airwave communications channels that could be detected by enemy eavesdroppers.

But more than three weeks after the first members of this battalion arrived in Saudi Arabia, the Marines only began digging their front-line defensive positions within the last few days. Commanders say they still have not received all the supplies they need to run their desert operation. Sandbags, the critical component of an infantryman's foxhole, have been in high demand and short supply.

The Marines say this battalion alone will need at least 90,000 bags to finish their fortifications. Thus far, they have received about half that number. About 1,000 to 1,500 bags are needed to build a single bunker, commanders said.

While many of the troops are shoveling sand by moonlight, Marine sniper teams are prowling the desert floor on reconnaisance missions.

As the Halloween-orange moon began to slide behind the distant horizon, Staff Sgt. Michael Barrett, 28, was doing "the sniper walk" back from his night patrol to the limestone quarry. He stalked crablike, hunched over with his bulging backpack, Remington rifle and night-vision scopes. "This is just one big danger area," said Barrett. "I wouldn't be caught out here during the day."

This night, however, brought few dangers. The Bedouin shepherd whose nearby home is a billowing white tent "didn't hit the rack till about 2300 {11 p.m.}, but his burro was excited all night and kept hawing -- or whatever they do," according to Barrett, a New York native.

A sniper team on another horizon peered through night-vision binoculars at a group of Bedouin women chatting on the sand until the early morning hours.

The shepherds, most of them Kuwaiti refugees who fled with their flocks of shaggy black sheep, appear unfazed by the military operations that have taken over the usually desolate desert. "We were concerned about them being within our lines," said Maj. Walt Casebolt, battalion operations officer. "But they've been very cooperative."

Casebolt said the Saudi who owns the limestone quarry where his unit is based introduced commanders to the Bedouins when they first established the camp. "We asked them not to let their livestock roam and to carry flashlights when they move around at night," said Casebolt, who said the Bedouins have complied with the requests.

In the early morning hours, before the temperatures have soared well over 100, troops practice offensive maneuvers that would be used if President Bush ordered the military to attack Iraqi troops in Kuwait. Hulking tracked vehicles that can operate on both land and sea spew massive waves of sand as they lurch across sand dunes and scrub brush.

The Marines assigned to the front lines endure some of the most austere living conditions of any U.S. troops here. They live in the hot, dusty quarry for about a week, then rotate to a Saudi work camp several miles away for two days of showers and air-conditioned barracks. Then, it's back to a week in the rock quarry and the desert bunkers.

It's the little amenities that Marines say they miss most. "We've been out of 'dip and chew' for days," said Chief Warrant Officer Chuck Whittlesey, the Marine Corps' top chemical warfare protection specialist assigned to train troops here. "A soda is a blessed event -- a cold soda would be a miracle."