FORT IRWIN, CALIF. -- Before a shot has been fired in the confrontation between Iraq and the United States, one of the most contentious skirmishes affecting the future of the U.S. military is already underway here in the Mojave Desert.

For the next month, the 48th Infantry Brigade -- a National Guard combat unit from Georgia -- will try to prove it is worthy of fighting shoulder-to-shoulder with an active duty division already in Saudi Arabia. The Guard's training and testing will conclude with a series of intense tank battles against the "Sumerians," a crack unit of professional soldiers who will defend an intricate network of Iraqi-style trench fortifications now under construction.

At stake, beyond the reputation of a unit first organized in 1825, is the "total force" policy adopted by the military in 1973 after conscription was abolished. By interweaving active duty forces with guardsmen and reservists, the military wanted to ensure that no sustained conflict could be fought without the reserves. The idea was to draw civilians into the fighting force in order to force the White House to gauge national resolve early in any future military crisis.

"This is like a 4,800-man football team, each member of which has received some training in blocking and tackling or calling signals. Now we've got to make a Super Bowl champion out of them. But it's not a game. The sports analogy breaks down because it's a deadly serious business," said Brig. Gen. Wesley K. Clark, commander of the National Training Center (NTC) and the man responsible for the Guard's training here.

"It's all we can handle," added Brig. Gen. William A. Holland, the 48th commander who manages a Georgia carpet plant in civilian life. "Very complicated and humongous."

Tens of thousands of reservists have already deployed to the Persian Gulf as part of Operation Desert Shield. But most are rear area support troops, serving in supply, transportation and medical jobs that often are similar to their civilian professions. Front-line combat troops require a degree of physical fitness, synchronization and technical competence difficult to maintain in Guard units like the 48th Brigade that only practice their soldier skills about six weeks a year.

Thus, when the 24th Mechanized Infantry Division -- which the 48th is supposed to supplement, or "round out" in military parlance -- was shipped to Saudi Arabia in August, the Guardsmen were left behind. "There was a tremendous disappointment," said First Sgt. Dempsey Logue, who normally works for a construction company in Savannah. "We'd always been led to believe that when the 24th was sent, we'd go with them."

The brigade may get there yet. The Army will make no decision on whether the 48th is ready to fight until after desert training ends Feb. 14. But some military officials believe the Pentagon will be so desperate for combat units in the gulf -- and under such pressure to demonstrate that the round out concept works -- that the Guardsmen are virtually certain to be certified as battle ready. If a ground war has begun, the brigade might even be flown to the Mideast and outfitted with surplus equipment rather than wait two months for their own M-1 tanks to be shipped from Georgia.

Reserves are now integral to the U.S. military, with half of the Army's combat punch and two-thirds of its support and service capability in the reserves or Guard. Because such units are cheaper than active forces, some members of Congress would like to give the Guard and reserves more responsibility as the United States shrinks and restructures its armed forces.

But many active duty officers privately worry about the combat readiness of the part-time soldiers. Some Guardsmen, particularly the non-commissioned officers, tend to be older than their active duty counterparts. Logue, the first sergeant from Savannah, is 58, having served in the Guard for 41 years without ever being called to duty before. The unit's top non-com, Sgt. Maj. Wesley H. Sheppard of Dublin, Ga., is 54.

"Yes, there are some reservations over whether I should be here at 54," Sheppard said. "A younger man might be better. But I'm hanging in there, trying to take care of my troops."

A General Accounting Office study in 1989 found numerous deficiencies among some National Guard units, including inadequate training and a lack of critical equipment such as night vision goggles. Rep. Les Aspin (D-Wis.), chairman of the House Armed Services Committee, has called Operation Desert Shield "a laboratory test" to determine whether reservists are adequately trained and capable of fighting.

Nowhere is the laboratory more intense -- or colder -- than here at Fort Irwin, a 640,000-acre swatch of the Mojave where the 48th Brigade arrived on Jan. 4 during an unusual bout of cold, rain and fog. Before leaving Georgia for a tour of duty that could last up to two years, the troops made out their wills, drew their equipment and marched in send-off parades staged by many of the three dozen towns that provide units to the brigade. Some of the officers stopped at Fort Leavenworth, Kan., on the way to California for a cram course in tactics.

For the first few days, the Guardsmen have been working on basic platoon skills -- how, for example, a tank platoon should fight off an infantry ambush with machine guns while also repulsing an attack by four enemy tanks. "Any unit will win if they're really good at platoon level," said Col. Greg Camp, one of the resident instructors.

Much of the training has Iraq distinctly in mind. Tank companies will practice "breeching" techniques -- how to blast through the elaborate complex of barbed wire, minefields, ditches and earthen berms built by Iraqi forces across Kuwait and southern Iraq. Infantry companies will rehearse trenchline assaults and how to clean out the fortifications once the armor spearhead has broken through.

The Guardsmen have learned that open desert terrain requires defensive positions flush with the ground rather than "Taj Mahal-style" above ground; that a two-man foxhole may require filling 240 sandbags; that "buttoning up" -- remaining inside tanks and armored personnel carriers -- is the best way to survive Iraqi artillery barrages.

As the training advances, the brigade will begin "force-on-force" maneuvers against the Sumerians, a resident force considered one of the toughest units in the Army. Large Iraqi fortifications, with a network of six-foot-deep trenches stretching three miles across the desert floor, are being readied. Regular troops already have conducted assaults against the complexes as part of an intensive Army study; a dozen generals attended a "counter-obstacle conference" here earlier this week.

NTC officers said the Guard brigade has discovered plenty of kinks to work out -- from maintenance snafus to supply problems -- but the prospect of imminent combat has given the training a focused intensity. "To a man, we think we're going to war," Logue said. "We've told ourselves that we're going and if we don't it will be a blessing."

After a week in the desert, morale remains high. Many Guardsmen feel they have something to prove.

"There's a desire to get away from the weekend warrior stigma," said Capt. Warren Johnson, 28, a university administrator who commands Echo Company of the 1st Battalion. "Nobody wants to be thought of as the old Guard. We want to be thought of as wearing the same uniform as everybody else."

Staff researcher Lucy Shackelford in Washington contributed to this report.