The F-15 fighter powered its engines to a thunderous roar and launched into the air, climbing at a steep angle and banking sharply to the right. Within seconds, it had shot so far up that it was reduced to a gray speck against the clouds as it soared off to Iraq.

A few hundred miles to the north, tanker planes lingered near the border, ready to refuel warplanes to give them more patrol time over southern Iraq. They were coordinated by the clunky AWACS radar planes with the distinctive dishes on top.

"It's a busy airspace here," said Maj. Gen. Saeed al-Haznawi, the Saudi commander at Prince Sultan.

Even as U.N. arms inspectors set up shop in Baghdad in an effort to head off a violent confrontation, the United States has already begun the war before the war in a deliberately noisy way: moving more equipment and troops into the region, staging exercises in Kuwait near the Iraqi border, dropping propaganda leaflets urging President Saddam Hussein's supporters to betray him and bombing targets within Iraq whenever given an excuse by Iraqi air defense units.

The activity serves not only to build allied forces and to soften up Iraq's defenses in case real war comes, but also to turn the screws on Hussein at this moment of decision. The more the Iraqi president feels encircled and sees bombs falling on his territory, some officials and analysts in the region believe, the more likely he might be to comply with the U.N. inspections.

"It's part of the psychological game to put pressure on him, to keep everybody convinced that they mean business," said Prince Turki Faisal, a senior member of the Saudi royal family and the country's former intelligence chief. He noted that the strategy "has borne fruit in the sense that it convinced Saddam Hussein" to readmit the inspectors.

So far, most U.S. allies in the region have cooperated, permitting U.S. forces access to what they need, in part because they hope to persuade Hussein to back down and avoid war. But countries around the region are beginning to prepare for the fallout in case the strategy does not work.

In Riyadh, the Saudi capital where three dozen Scud missiles fell during the 1991 Persian Gulf War, air raid sirens have been put back up. In Kuwait, schools have been rehearsing evacuations and the government has ordered 2 million gas masks. "This region cannot take another war," lamented a Saudi official who asked not to be identified.

As the Bush administration canvasses 50 allies around the world to see how much support they would offer in case of war, few places are more critical, or sensitive, than Saudi Arabia. From here at this expansive air base located in the flat, barren Arabian desert about 70 miles southeast of Riyadh, the U.S. military manages air operations in the southern "no-fly" zone -- and could run the next war.

Every day, pilots return from Iraq, often with reports of being fired upon by antiaircraft guns, which prompts retaliation. U.S. warplanes patrolling the southern no-fly zone have been shot at during nine of the 12 days since the U.N. Security Council passed its resolution mandating new inspections, according to the Pentagon, usually resulting in retaliation. Just this morning, coalition aircraft bombed three Iraqi air defense communication facilities near Al Kut and Basra.

"The pilots flying patrols in the no-fly zone are essentially flying combat missions, given the circumstances," Rear Adm. David A. Gove told reporters at the Pentagon. "Any opportunity that they have to understand the capabilities and the layout of Iraqi air defense weapons systems is useful for their own experience base. And there has been degradation of the integrated air defense system in Iraq."

U.S. commanders generally dispatch planes from Kuwait out of deference to Saudi sensibilities, allowing Riyadh to maintain that it has not allowed attacks on fellow Arabs from its territory. For U.S. war planners, however, the attraction of Prince Sultan Air Base is not the line of F-15s parked here but the state-of-the-art Combined Air Operations Center. Located in an anonymous, low-slung building in the middle of the base, the center can process extraordinary volumes of data, from satellite imagery to reconnaissance from surveillance planes to reports from the field, at the same time that it tracks hundreds of aircraft operating across the region.

Out of fear that they might not be allowed to use the center to direct a war against Iraq, U.S. military planners built an alternative center in Qatar, but still would much rather use this site. As one U.S. official put it, the Qatar center is a backup at best, akin to a thin spare tire that cannot travel as fast on the highway.

Saudi officials have sent mixed signals about whether they might allow the base to be used for a war against Iraq. "Saudi Arabia will do what's in its best interests and in the best interests of its friends and allies, including the United States," Prince Khalid bin Sultan, the assistant defense minister, said in an interview. "Let's not forget what it did in '91. . . . We shared our blood together. We protected each other in the field and we saved American lives as America saved other countries, including Saudi Arabia."

Still, he noted, that was for the clear purpose of liberating Kuwait from an Iraqi invasion. "This situation is different," he said.

If the moment arrives, U.S. officials expect the Saudis to at least permit the use of the command center and grant overflight rights for attack planes flying from bases elsewhere. They also want the right to base AWACS planes here, along with tankers to refuel attack planes in Saudi airspace before they head into Iraq.

The presence of 4,000 U.S. troops, along with British and French forces, has been provocative in the homeland of the prophet Muhammad, one of the main grievances that has animated terrorism suspect Osama bin Laden, a Saudi national. But as Riyadh labors to repair damaged relations with Washington following the Sept. 11, 2001, attacks, it has shown new signs of openness about U.S. military operations at the base.

Prince Khalid allowed a reporter to visit the base Tuesday, in a rare moment of access this year. However, U.S. military commanders, who have long cited Saudi sensitivities in barring journalists, declined to permit the correspondent to visit their portions of the base, including the command center, or to interview U.S. troops.

Maj. Gen. al-Haznawi, the base commander, showed no reluctance to put the Saudi partnership with the United States on display. "It's one team really," he said. "We have no problem with the way we work together."

In this isolated locale outside the farm town of Al Kharj, Prince Sultan Air Base sprawls over 230 square miles, all open except for a handful of palm trees and dried brush, making it easy to protect against attacks. The main gate is 12 miles from the central administrative building. Many U.S. forces were moved here after the 1996 terrorist attack on the Khobar Towers military housing complex in Dhahran, 225 miles northeast of here, killed 19 U.S. service members.

The Americans live in their own cordoned-off part of the base in two-level dormitories along with stores, baseball diamond, swimming pool, Internet cafe and an ice cream shop. The Saudi section is speckled with mosques.

Construction equipment can be seen at sites for new air-conditioned U.S. housing, but from the Saudi portion of the base the signs of the war before the war are hard to detect, evident mainly in the thunder of aircraft as they take off or the tight spiral of tanker planes as they land.

Inside the fenced-off command center, though, officers are guiding the beginning stages of what could become a far bigger conflict. "It's not going to be a cakewalk," warned Prince Turki.

As he works with the Americans, al-Haznawi makes a similar point. "It will be different than Afghanistan. It will be different than the '90s," he said. "Every war is different than the last one."