If Iraqi troops cross the border from Kuwait and head for the world's largest oil fields, the desert kingdom of Saudi Arabia may be forced to use the tens of billions of dollars worth of military hardware it has purchased from the United States, Europe and even China over the past decade.

The Saudis have managed to avoid warfare in the region by using their oil billions both to placate potential enemies and to buy deterrence.

They have stockpiled sophisticated planes and missiles and command and control networks because "they have a huge territory and a small population. They feel very vulnerable and live in a rough neighborhood," Walter Cutler, who was U.S. ambassador there twice during the 1980s, said in an interview yesterday.

The key to the Saudi defense is its air force -- which has an intricate command network including AWACS radar planes, and commitments that the United States would come to its aid if attacked.

President Bush has said Saudi Arabia is one of the "vital interests" of the United States, and administration officials have made it clear any move by Iraq against Saudi Arabia would be met by a U.S. military response.

The U.S-Saudi relationship has been especially sensitive in recent years. A strong pro-Israel Congress has blocked arms sales to Saudi Arabia, sending King Fahd to other suppliers. The Saudis, at the same time, deceived the United States in buying Chinese-made missiles in 1988. And they have not offered the United States basing rights, although major military bases at Dhahran and Riyadh were purposely built bigger than needed to enable them to accommodate U.S. planes in a time of crisis, according to military experts.

"All the air bases are oversized, designed to take large U.S. transport planes," said Mazher Hameed, a Persian Gulf security specialist. "There are underground hangars at Riyadh and Dhahran. . . . All that capacity has no conceivable use but for U.S. forces."

Cutler said that during the eight-year Iran-Iraq war, the Saudis were concerned fighting might spill onto their territory. But most of the thinking was about how to respond to a lightning-quick raid from Iran, not preparation for a massive armored attack from Iraq, he said. After all, Saudi Arabia was helping finance Iraq's war and carrying Iraqi oil in new pipelines.

Anthony Cordesman, another gulf security expert, said last night that Saudi ground forces are ill-equipped to face an armored attack. He noted there are only 66,000 men in the Saudi military, about half in the national guard. The army has no combat experience and is scattered around the country -- which is roughly the size of the United States east of the Mississippi River.

About one brigade of army troops is stationed at King Khalid Military City, located near Hafar Al Batin, southwest of Kuwait and closest to the Iraqi troops, Cordesman said. The military installation was built from scratch starting in 1979 and was "specifically designed to check any kind of advance from the northeast corridor, from Iran or Iraq," Hameed said.

Cordesman said that half the Saudi force of about 500 tanks are lightly armored French models, which could not stand up against Iraq's Soviet armor. But the air force has plenty of U.S.-made Maverick anti-tank missiles.

The main air base at Dhahran protects the kingdom's main oil export facilities on the gulf coast and is home to some of the Saudis' 62 F-15 fighters and a growing number of British-built Tornadoes, bought when Congress rejected the idea of more F-15s in 1985.

Cutler and Hameed said the Saudis' first line of defense would be protecting its major bases from air attack, though they also would face the possibility of Iraqi missiles being fired at refineries and oil storage tanks.

Dhahran and the country's largest oil fields are about a four-hour drive from the border with Kuwait, Cutler said. If the Iraqis crossed the border, Hameed said, the Saudis would sever the main highway, trying to force armored columns into the desert.

Cordesman agreed the most likely avenue for an Iraqi advance would be down the coastal highway. "That is most vulnerable, and also easiest to defend," he said, noting the Saudis also have many ground force anti-tank weapons.

The Saudis "have never treated their army as well as the air force. Now might be a good time to do so," he added.

If Iraq attacks, "Saudi Arabia doesn't have the capability of operating on its own," Cordesman said. "The assumption has always been that in a true emergency they would get American aid."

The United States has massive airpower resources to draw on to help the Saudis, and such assistance would be backed by the command and control networks called "Peace Shield" as well as the AWACS, Cordesman said.

U.S. options, he said, would include flying in two wings of Air Force fighters, or 144 planes; sending in Marine short-takeoff planes; using aircraft carriers offshore; basing bombers in Diego Garcia to carry out raids or even using conventional cruise missiles and B-52 bombers.