WITH U.S. FORCES IN SAUDI ARABIA, OCT. 27 -- After two months in the desert, U.S. Marine Lt. Jerry Curby and his men seemed to know the nighttime desert assault drill by heart.

Scrunched inside three amphibious assault vehicles, they scurried across the moonlit desert at 20 mph on night security maneuvers. At Curby's orders, the 26-ton treaded vehicles braked, their tail ramps lowered, and the Marines fell out to take assault positions in the sand.

The mood was disciplined but relaxed, partly because there was no "live fire" from the enemy, and partly because these men belong to one of the first contingents rushed to Saudi Arabia after it asked for U.S. military protection from Iraq's occupation force in Kuwait.

Most of these soldiers of the 3rd Marine Regiment's 1st Battalion, Alpha Company, based in Hawaii, were marking the end of two months in this country, settling in for what they now realize may be a longer stay than they anticipated.

While they seemed in relatively good spirits, their major complaint to a group of visiting reporters was not the 10-day delay in getting mail, or the dearth of outside news, or the possibility that public support at home was ebbing. More than all these, they complained about the uncertainty.

"We don't know when we're ever going to leave," said Cpl. Patrick Womble, of Washington, D.C. "What bothers me the most is we just don't know what's going to happen. One day we may be training. The next day, we may be in Kuwait."

The U.S. troop buildup here began in early August with the frenzied, gung-ho gusto of an invasion. U.S. transport planes landed minutes apart, sometimes disgorging 10,000 troops a day, according to the Army's logistics chief, Maj. Gen. William Pagonis. Scores of ships unloaded equipment; air defense systems were assembled; nearly 60 offices and warehouses were rented; field hospitals were set up; soldiers were sent to desert camps. Adrenaline ran high.

So far the 200,000 U.S. troops based on land and at sea in the Persian Gulf, along with thousands more from other Western and Arab nations, have accomplished their stated mission: to protect Saudi Arabia from an Iraqi assault. But the pace of arrivals here has slowed, even though officials in Washington this week discussed the possibility of sending 100,000 more troops to the gulf region.

Col. Rick Kiernan, a public affairs officer in Riyadh, pulled out a diagram he had made to show just what has happened: Deployment has been replaced by "sustainment." This is a time to deal with tasks like sorting 50 tons of mail a day, not to mention sorting out who will do what if fighting breaks out. It is also a time of waiting -- a time, Kiernan said, that is critical for troop morale.

Officers in the field, like Marine Maj. John Bates, executive officer of the 1st Battalion, know this too. Asked what his biggest obstacle is, Bates replied: "The uncertainty. Just waiting for the word. . . . For the first few weeks, we thought we'd really get hit {by the Iraqis}. In the second week, we thought we'd go and liberate Kuwait.

"And now, we're just awaiting word."

Morale appears to vary among U.S. military units here, depending on how much mail, hot food and outside news they get; those camped in the desert receive very little fresh news. Capt. Michael Huber, commander of an airborne engineering company out of Fort Bragg, N.C., said that while morale among his men based close to town was "excellent," he had a "platoon up north that's got it kind of rough because they're out in the hot sun with no relief at all."

Sometimes the shock of their new surroundings also makes morale plummet. Army specialists Patricia Gallier and Tamara Davis used to run computers with the 8th Ordnance Company at Fort Bragg. Last week, they were sweating as they shoveled sand into bags outside their new quarters, a fenced-in, dusty patch of earth where they share a tent with 15 other women.

"The worst part is being shut in and not being able to go home at night," said Gallier, who lived off-base with her husband in North Carolina. "In all my life I never believed I'd be in Saudi Arabia, filling sandbags in the hot sun."

Keeping up the troops' spirits is not easy in a country where alcohol, bars, dancing, movies and opportunities for social interaction with the opposite sex are proscribed. Most of the American forces here are confined to their tents, whether in the desert or urban areas, even after hours. Reading, watching videos and writing letters are the most exciting recreation they find. Even on the base, beer is banned.

"We had a good argument the other night about the constitutionality of defending a country that doesn't allow beer," an Army sergeant recently groused.

To keep his Marines' morale up and boredom down, 1st Battalion executive officer Bates said he "keeps busy, with an objective. It's really by design that they have very little free time."

For six days, his men are in their desert camp. Breakfast and dinner are hot meals, brought out by a chow truck. Lunch is military field rations known as Meals, Ready to Eat, or MREs. Since no lights are allowed at night, bedtime comes early. Morning exercises begin around 5:30 a.m.

Each six-day period is broken by three days in a more comfortable rear camp, where the men can shower, wash clothes and phone home.

So far, this battalion had done most of its training, both defensive and offensive, at night, officers said. But on Friday, Capt. John Carretti's Bravo Company practiced a daylight attack on a trench dug by Marine engineers. Bravo Company stormed the trench from a nearby hill of sand three separate times, yelling "butter, butter, butter" to simulate machine gun fire.

Debriefing his men afterward, Carretti said the exercise had gone well, but told his troops they had to move faster. Communications, he noted, had also been a problem.

This morning after breakfast, Marine Division Chaplain Stanley Scott visited the camp to brief a weapons company that left today for a five-day stay with Saudi troops stationed in the desert.

It was to be these Marines' first interaction with their Saudi hosts. "We're going up there to do our training. . . . If they're interested in participating, we'll let them," said the company's commander, Capt. Michael Callaghan. Although combined exercises with Saudi troops are a stated aim of U.S. military commanders in preparation for future military action, very few have been conducted so far, according to military officials.

Scott, who is known here as a "spiritual adviser" rather than a chaplain in deference to Saudi Arabia's Islamic society, gave a short course on Middle East history, Islam and Saudi customs. He emphasized the importance of hospitality, explained why Saudi women veil their faces and warned his audience that when they see Saudi men greeting each other with kisses on the cheek, it does not mean they are homosexual. In their culture, he said, "it's a sign of close friendship."

At the end of his talk, the Marines got further instructions from a sergeant: Leave behind playing cards and any MRE with pork because both are offensive to Saudi Moslems.

As with all U.S. combat forces here, the Marines' desert training is meant to prepare them for the worst. But for many at an uncertain time such as this, the waiting game overshadows the war games. Danger seems to recede and the worst can be forgotten.

"We're just going to gather up our dead and head for you," one Marine radioed to his buddies at the end of Friday's simulated trench attack.

"They're funny," said one of the listening Marines, amused at his friend's effort to make the exercise seem real. "Well, it's funny now."

"It won't be later," added another Marine.