WITH U.S. FORCES, SAUDI ARABIA -- Suddenly it has become real.

U.S. Marines entrenched in the chill, sandy expanse of northeastern Saudi Arabia have begun approaching their duties with a new attitude in recent days. The sleek warplanes screaming overhead, the stench of a burning oil refinery hanging in the desert air, the dull rumble of distant bombs and rockets -- the signs have multiplied that real combat may soon be in the offing.

For the first time in most of the young Marines' lives, it will not be the make-believe of training in the United States or even the five months of drills since U.S. deployment began in the Persian Gulf crisis. It certainly will not be the casual heroics of "Rambo" movies that can be turned off at will.

The realization is dawning with increasing intensity on the thousands of Marines deployed south of the Saudi-Kuwait border that the struggle they are approaching is the kind you may not come home from, or in which your friend could be killed or maimed in a bloody patch of sand miles from nowhere.

"We all know there are going to be some casualties, some deaths," said Lance Cpl. Carlos Morales, 20, of the Bronx, "but we try not to think of it. We just try to live day to day."

For Morales, a combat engineer, living day to day has meant sleeping under a tent dug into a hole scooped out of the sand by Marine bulldozers. On the frequent rainy days, it has meant a damp sleeping bag and, if his tent happened to be set up in a spot of rock or clay, a puddle of icy water for his immediate surroundings.

It has meant guard duty through the night, when the north wind sweeps unhindered across terrain so desolate that even a sandbagged bunker seems friendly by comparison. Peering into the darkness, Marine sentinels have grown used to pinpoints of light from supply trucks relentlessly moving across the desert under blackout rules, and to sudden flashes on the horizon that herald an Iraqi missile crashing down somewhere.

Marine units encamped near targets such as air fields or oil refineries have been roused several times a night by sirens alerting them to missile attacks since U.S. and allied bombing began Jan. 17. Hunkered down in their bunkers, they often have felt the ground shake and the air vibrate as Iraq's Brazilian-made Astro or Soviet-designed Frog missiles slam into the ground nearby.

As is the case with the Scud missiles fired at more distant targets in the Saudi cities of Riyadh and Dhahran, however, the medium-range Astros and Frogs so far have failed to land where they could do significant damage. Marine bases, with this in mind, have spread out over miles and miles of the seemingly limitless desert, leaving swaths of empty sand between soldiers' tents, supply dumps and the all-important chow hall.

Morales stood with his platoon on one such stretch of emptiness the other day, getting another lesson on the looming reality of what could rapidly become a large-scale ground assault on Iraqi defenses on the other side of the close-by Kuwaiti border. The lesson was mine detecting, and the Marines were paying attention. Failure to pay attention has taken on higher stakes: from demerits to the possibility of death.

"All you Rambos who've got knives and other garbage hanging from your gear, I don't want to see that garbage any more," shouted Staff Sgt. Rick Taylor, who was demonstrating use of the classic mine detector disk on the end of a long handle. "Because what's going to happen? It's going to fall off, and then what? It's going to set off a mine. And if that happens, I'm going to crawl over with my bloody stump and choke you out."

The diatribe, which might have brought guffaws six months ago in some U.S. garrisons, generated a few nervous laughs. But mostly the Marines listened intently and carefully watched Taylor as he twisted at the waist to pass the disk back and forth over the sand.

"They're asking a lot more questions and paying a lot more attention," Cpl. Christian Brookins, 25, of Orlando, Fla., said of his squad.

As Brookins chatted with a visitor, the instructor told his students that everyone in the squad had to know every job in the unit: detecting, probing, security backup from an M-60 light machine gun.

"If half of this squad is taken out, somebody's got to be the new sweeper," he explained.

Lt. Chris Simmler, 23, of Franklin, Mass., said his company of combat engineers had received mine detector training in the United States, but the lessons were being repeated, he said, in the new atmosphere of impending combat that increasingly infuses the ranks.

"Our big thing is psychological right now," Simmler explained. "We've got to get them confident. We've done this in garrison mode back in the States. Now it's for real. There's not so much joking or fooling around."

Because so many years have passed since U.S. forces left Vietnam, only a handful of the thousands of Marines deployed here have seen the bloodletting likely in an all-out assault on occupied Kuwait. "Our 'top' is basically the only one around who's been in combat," Brookins said, referring to the unit's top sergeant.

Perhaps for that reason -- or perhaps because it is being hidden from view -- there is little sense of dread at what might lie ahead. Young corpsmen who soon may be called on to help decide who has a chance to live and who is best left to die as casualties come in seem intent only on their training. Lt. Cmdr. Keith Boehm, 42, of Virginia Beach, Va., introduced them to visitors at a forward field hospital the other day.

"I've never been in a war, but we know stress is one thing, fear another, or feeling isolated," Cpl. Morales said in seeking to describe the stoicism displayed by many Marines. "We've never experienced these things, but we know what they are."

While confidence in training has become familiar in the Marine units girding for battle, at least two Marines in the field now say they oppose the war and have applied for conscientious-objector status that would allow them to go home before the fighting begins. Another, wounded by an M-16 round fired into his knee, is being investigated to determine whether it was an accident, as he contends, or whether he deliberately shot himself to avoid battle.

Confidence also has been high in the ability of U.S. weaponry to pummel Iraqi forces in Kuwait so hard that a final assault by combat troops would be overwhelming. Simmler said his combat engineers and their mine detectors, for instance, would only be a "last resort" after airplanes, artillery, mortars, line charges and bulldozers had already exploded all but a few of the mines believed planted all along the multi-tiered Iraqi defense lines.

Similarly, officers have expressed expectations that many Iraqi defenders will crumble and surrender under what is expected to be a traumatizing bombardment in the final stage of the allied air campaign. Several barbed-wire compounds have been erected for prisoners of war along the Marine emplacement just out of Iraqi artillery range.

"They're saying a lot of troops might surrender, so they might just let us right through there," Simmler said.

Officers' tents have been hung with maps showing in detail Iraq's defensive emplacements, staggered in deadly layers from the Saudi border northward 60 miles to Kuwait City. The maps, the fruit of months of satellite and airborne reconnaissance photography, give Simmler and higher-ranking commanders a clear picture of the barriers they must destroy if Iraq's soldiers are to be expelled from Kuwait.

However, many Marines betray a striking lack of clarity about the country whose forces they are likely to grapple with. Deployed for months in the puritanical Saudi kingdom where women are veiled, alcohol is forbidden and Jewish or Christian soldiers have to worship discreetly, they have shown surprise to learn that in Iraq they could order a beer, attend church or synagogue openly and see women's legs.