The military problem of defending Saudi Arabia and several of its oil-rich neighbors and preserving the flow of oil through the Persian Gulf has grown in magnitude with each day of the Iraq invasion crisis and shows no sign of abating.

The military operation began as an exercise of air and naval power, but has evolved to a mixture of air, land and sea forces whose multinational command structure was still being negotiated at the Pentagon yesterday.

By the end of the month, the United States has contingency plans to deploy up to 50,000 or more ground troops to the desert kingdom to join an equal number of Saudi ground forces, and a still undefined number of troops from other countries including Pakistan, Morocco, perhaps Egypt and other nations who are reported to have committed forces but not announced this publicly.

Some military officials and analysts express concern that U.S. leaders do not fully understand the risks of what may turn out to be a military deployment that dwarfs both the Panama invasion force and the Persian Gulf escort deployment of 1987-88.

If forced to confront an Iraqi invasion into Saudi Arabia, this new multinational force would face a battle-tested Iraqi army that is now maneuvering more than 200,000 soldiers in an area of operations from southern Iraq into Kuwait and down to the Saudi border.

"{The Iraqis} conducted the Kuwaiti operation in a very professional manner. It's an army that is capable," said Gen. Colin L. Powell, chairman of the Joint Chiefs of Staff, at a Pentagon news conference. "But they are not invincible, and they're not 10 feet tall. And we have the capabilities that we are now bringing to bear to ensure that we, in concert with the armed forces of Saudi Arabia, can deal with that threat."

Military officials said Iraq has moved up Soviet-made Scud-B missiles within range of the airfields U.S. and Saudi aircraft will need to use to operate in any military conflict. "We have nothing to shoot those down," one official said, raising the possibility that the Soviet-made missiles could preemptively punch huge craters in Saudi runways and deny the Saudis and Americans the forward bases they would need to control the air over the battlefield.

The 300- to 500-mile range of the modified Scud-Bs in the Iraqi arsenal, and their potential to carry either a 2,000-pound explosive warhead or a lethal load of poison gases, puts at risk major Saudi cities, oilfields, refineries and military bases, officials said.

Both President Bush and Powell highlighted the threat to Saudi Arabia from missiles yesterday. "If you look at the {missiles'} range arcs, that is one of the principal threats directed against Saudi Arabia," Powell said.

But it is not the only threat.

Iraq has demonstrated the capability to drop ship-killing Silkworm missiles from Chinese-made heavy bombers and fly their French-made Mirage fighters above the waves to launch deadly Exocet missiles. They also learned from their eight-year war with Iran that mine warfare is a cheap and effective weapon against modern navies.

A senior Navy official yesterday said U.S. aircraft carriers and other warships operating in the Persian Gulf area have for years tracked the Iraqi air force using long range radar and Airborne Warning and Control System (AWACS) surveillance planes. The official said the Navy is confident that it can defeat any Iraqi attempts to attack U.S. warships or U.S. and Saudi ground forces.

There can be accidents, however, other officials point out. The Iraqi attack on the USS Stark in May 1987 was a tragic example of technology defeated by misunderstanding -- the failure to challenge an intruder who appeared benign, but carried two Exocet missiles that killed 37 American sailors on board the destroyer.

"That's why we call it the fog of war," another analyst added.

Iraq's ground forces in Kuwait are backed up by armored columns of more than 500 tanks and 1,000 heavy artillery pieces that would concentrate fire in any advance. Saudi Arabia would have to mass its entire tank force to meet this threat, yet Iraq has more than 1,000 first-line tanks still in reserve. Trained and equipped by the Soviet Union, the Iraqi army uses an unconventional mix of heavy armored formations and more lightly armed and mobile Republican Guards capable of making fast flanking advances and infiltration maneuvers, according to U.S. and western military experts.

Not that the Iraqis are invincible. They are now strung out on a long supply line in a blistering desert -- a little like Napoleon's forces in Russia. To prevail in an attack against Saudi Arabia they would have to control the airspace above their advance, an unlikely prospect for their 500-plane air force against the best the United States, Saudi Arabia, Britain and others can offer.

Air power is a key element of the American military strategy to defeat this Iraqi force. But the American plan for confronting the formidable array of military challenges Iraqi President Saddam Hussein can bring into play -- including poison gas weapons -- is still evolving.

A direct confrontation on land was ruled out last week. As one Powell aide explained, it would take a 3 to 1 numerical advantage for U.S. forces to go on the offense against the Iraqi army.

The best way to stop Saddam, according to the initial Pentagon plan, was to concentrate U.S. and Saudi air power over the battlefield, striking Iraq's rear areas and supply line to isolate any invasion force and then swoop down and pick it apart with F-15s and other attack planes.

If Saddam escalated, if he used chemical weapons or if his armored thrust could not be stopped, more airpower could be called upon to deliver a withering blow: B-52s Stratofortresses flying out of Diego Garcia 2,100 miles away could be brought in to carpet bomb his formations, his industries and his oil installations in hopes of forcing the collapse of his offensive. Unpiloted cruise missiles launched from battleships at sea could also be used.

Since these plans were formulated in the Pentagon over the weekend, the contingencies have expanded. Yesterday Bush said the American commitment is to defend not only Saudi Arabia, but also "all those countries in the GCC {Gulf Cooperation Council}," the alliance that includes Saudi Arabia, Kuwait, Oman, Qatar, Bahrain and the United Arab Emirates.

The escalation in the American deployment will be paced by two factors: the change in the Iraqi threat and the formation of multinational forces. "We want Arab forces there, at risk, in front of our forces," one military official said. The U.S. escalation, according to several officials, will demonstrate to Saddam a building and unrelenting U.S., allied and Arab resolve to confront any further military moves of Iraq from occupied Kuwait.

If escalation is required, the U.S. Central Command could call on more ground and air forces that have been dedicated to just such a war scenario in the Middle East for more than a decade.

But at the outset of this still-evolving operation to defend the oil and nations of the Persian Gulf, the greatest concerns in the Pentagon are about the surprises that certainly lie ahead.