DHAHRAN, SAUDI ARABIA -- Hours after the acting regional U.S. commander arrived in Saudi Arabia, he asked his senior Army officer for an assessment of how much military power his service could immediately call upon to defend the threatened kingdom.

Army Lt. Gen. John J. Yeosock reached into his pocket, pulled out a tiny silver penknife, opened the blade and pointed it at Air Force Lt. Gen. Charles Horner, who was in command of the U.S. Middle East forces at the time. "Here you go, buddy. This is it," Yeosock recalled telling his friend.

Not any longer.

In the nearly seven weeks since C-day, the Pentagon's shorthand for the commencement of Operation Desert Shield, the U.S. military has amassed on the Arabian Peninsula the most sophisticated, high-technology array of weapons and equipment ever fielded in a combat zone by any nation.

On the opposite side of the Saudi border with Kuwait, Iraqi President Saddam Hussein, whose forces invaded the Persian Gulf emirate Aug. 2, has assembled almost one-half of his entire military -- one of the larger and among the more combat-hardened in the world -- along with an arsenal of surface-to-surface and ground-to-air missiles.

Standing between those two forces is a thin shield of Arab troops -- primarily Saudis -- which some U.S. military officials estimate could hold off an Iraqi invasion for no more than two to four hours before collapsing and falling back into American ranks.

With such a major buildup on both sides of the border, many U.S. military leaders say they believe it is virtually impossible to have so much hardware and so many troops facing each other without eventual combat.

When all of the forces that have been committed to Saudi Arabia -- including those from the United States, Europe and Arab nations -- arrive on this peninsula, more than 700,000 troops will be assembled in military zones along both sides of the border.

While Iraqi equipment is considered no match for that of the high-tech Western forces, analysts say, the sheer numbers of soldiers and tanks in Saddam's arsenal pose a formidable threat that cannot be countered by technology alone.

Military intelligence officials estimate that about 365,000 Iraqi troops have been moved into the region south of Basra in southern Iraq and into Kuwait. The number is expected to swell to about 400,000 in the next few weeks.

U.S. intelligence sources now provide the military with hour-by-hour reports of the movements of the Iraqi forces.

Since the invasion, Iraq has shifted its most qualified, heavily armored forces, the Republican Guard divisions, from the front lines to the rear, giving the military more power to counter initial blows to its front, now manned primarily by illiterate teenage conscripts, according to U.S. officials.

About 80,000 members of the Republican Guards were the shock troops in the Aug. 2 storming of Kuwait. But two weeks after the invasion, U.S. intelligence detected their withdrawal to the Iraq-Kuwait border region in what U.S. officials believe is an attempt to keep them in reserve as a quick-reaction "swing" force that could respond to assaults mounted either from Saudi Arabia by U.S. and multinational armored forces or from the Persian Gulf by U.S. Marines.

The remaining 60,000 Republican Guards are said to be concentrated in and around Iraq's capital, Baghdad, to protect Saddam and the Iraqi leadership from internal or external assault.

While the combat capability of the Republican Guards is highly rated, U.S. military and intelligence officials say much of the rest of the regular Iraqi army is roughly equal in combat readiness to U.S. National Guard troops. Nevertheless, the Iraqi army demonstrated a tenacious capability to defend its own territory during the eight-year war with Iran.

Iraqi military commanders -- much like their American counterparts -- reportedly are keeping many of their units highly mobile, rather than allowing them to dig into stationary positions, in an effort to give them more flexibility in the event of combat.

Military officials said they are beginning to see signs of trouble -- "hairline cracks" -- confronting the Iraqi military apparatus. Intelligence sources indicated that food is late in reaching the troops and that pay is slow to arrive. One official cautioned that the impact on Iraqi forces of such apparent problems is "hard to measure."

The Iraqi military has installed stationary and mobile Scud-B missiles across eastern and central Kuwait.

While American officials express confidence that they know the locations of all the stationary Scud launchers, military officials say the Iraqis play a constant game of hide-and-seek with the mobile launchers.

"The Scud is primarily a psychological weapon," said a general officer commanding U.S. forces here. "It is not an accurate weapon, but the fact that you could fire it and it would land somewhere in a city, killing innocent civilians, makes it more of a psychological weapon."

Some military officials here said they believe that some Scuds would likely evade U.S. air defenses.

On the Saudi side of the border, the combined Arab and Western forces have divided the Eastern Province and some other areas of the country into large sectors of responsibility, with Saudi and other Arab forces operating in zones immediately adjacent to the Kuwaiti border.

The first large units of American troops -- Marines and the Army's 24th Mechanized Division -- are located about 50 miles from the border defending important ports and oil refineries.

"We would like to be operating farther north, closer to the border area," said a U.S. troop commander.

The United States expects almost 200,000 military personnel to be in place within the next two months.

In addition to attack planes and fighter jets, Britain has committed its 6,000-member "Desert Rat" armored brigade.

The Saudi military has about 65,000 active-duty troops and is urgently recruiting men for its guard and reserve units. Egypt has said it could send up to 30,000 troops to Saudi Arabia and the Syrian government is planning to dispatch about 18,000 troops.

Although the buildup inside Saudi Arabia remains almost two months away from completion, enough air, sea and land forces have arrived to erode the overwhelming and undisputed military advantage Iraq held in the days following the invasion of Kuwait.

Throughout Saudi Arabia's Eastern Province, where the vast majority of the military forces are concentrated, there is a combat-zone atmosphere.

Long convoys of trucks piled with equipment or loaded with tanks snake along the main highways from the massive Saudi ports to staging points in the surrounding desert. Helicopters, jet fighters and cargo planes criss-cross the skies around the clock. Military personnel clad in desert camouflage gear have filled many of the hotels in major cities, turning hotel restaurants into military mess halls.

The United States has moved some of its most sophisticated weaponry to the region, including F-117 Stealth fighter jets. They could slip past Iraqi air defenses, regarded as equivalent to U.S. systems of the Vietnam era, and conduct night attacks. Patriot missiles have been extensively deployed to protect critical airfields, ports and other strategic locations from attack by Iraq's Scud-B missiles and aircraft.

The complete array of warplanes assembled in the region by the United States, Saudi Arabia and Britain is formidable. But Army and Marine Corps commanders are quick to note that the United States has never won a war on air power alone.

Military officials consider the heavy combat units now en route to Saudi Arabia critical to any battle, explaining the uneasiness some senior commanders still feel about the shortage of combat power now on the ground in Saudi Arabia.

Combat, initiated by either side, would most likely begin during the night. Critical to both forces will be night-vision equipment.

After eight years of conducting night operations in the Iran-Iraq war, Iraqi forces are thought to be proficient in the use of night-vision devices, and their tanks and weapons are equipped with them.

American forces consider their equipment far superior, however, and have dedicated much of their training to night operations.

In the vast open spaces of the desert, the force that spots its enemy first will have the clear advantage, U.S. officials say, adding that their sophisticated aerial reconnaissance and other technology will give their eyes much greater range than the Iraqis'.

U.S. miltary officials consider Saudi and other Arab forces lining the border merely as a tripwire for any advancing Iraqi forces.

"No one views them as the force to stop an Iraqi attack," said one U.S. commander. "Their job is to screen and defend. We would take them into our lines and go from there."

If Iraqi troops moved across the border into Saudi Arabia, the United States would unleash its heavy M1 tanks, Apache attack helicopters and A-10 tank-killing planes against the Iraqi ground forces. Some territory would have been mined to impede advancing forces.

Meanwhile, the 82nd Airborne or special forces Ranger units presumably would be dropped behind enemy lines to sever supply lines and sources. Special forces units now operating near the border with special electronic jamming equipment would attempt to block radar signals and foil Iraqi aircraft.

In addition, U.S. aircraft would have begun bombing strategic points in Iraq, blasting airfields so Iraqi pilots would have no place to land, according to officers.

Other plans call for bombing other strategic points throughout Iraq, including President Saddam Hussein's governmental offices and residences.

U.S. military leaders concede that any major ground battle would result in large-scale casualties on both sides. The Pentagon has dispatched medical units equipped with thousands of beds to the region, including two massive naval hospital ships. It has also ordered mortuary units to the region.

In battle, one of the most critical elements will be communications among the more than 20 nations that have joined in the defense efforts of Saudi Arabia.

After seven weeks of operation, joint efforts still remain minimal. The United States and Saudi Arabia have exchanged liaison officers with most units now in the kingdom, and small U.S. special forces teams are conducting weapons training with some Arab forces, officials said.

Still, battlefield communications are something that even NATO forces -- after 40 years of joint exercises -- have not fully mastered.

"It will be a challenge," said one U.S. general commanding troops here.

Staff writer Patrick E. Tyler, also in Dhahran, contributed to this report.