KING FAHD MILITARY CITY, SAUDI ARABIA -- Their knees were a little shaky, and standing at attention on one leg in the hot sun reflected off the asphalt parade ground was a difficult balancing act many couldn't hold for long.

But their spirits were high and a clear sense of determination was written on their soft civilian faces as they went through their second day of basic drill to the bark of an understanding sergeant.

These are the "instant soldiers" of the Saudi army's brand-new reserve force, one step in the long-term expansion of the armed forces that the government has decided upon in response to the threat of Iraqi aggression.

Iraqi President Saddam Hussein's occupation and annexation of Kuwait have prompted a radical rethinking about the kingdom's enemies and defense needs as well as an acute awareness of its vulnerability to an Iraqi ground attack.

Saudi officials are talking about tripling the 38,000-man army and doubling the air force, which now numbers 16,500 personnel and 180 combat aircraft, according to the London-based International Institute of Strategic Studies.

Some officials envision the formation of six or seven divisions armed with well over 1,000 tanks to create a credible deterrent to what they see as a probable long-term threat from Iraq. The bulk of the Saudi tank force now consists of 300 aging French-built AMX30s and 250 older-generation American-made M60s, which are to be augmented soon by 150 updated M60A3s.

But the Saudis have signed a letter of intent to purchase 315 modern M1A2 tanks from the United States and now are considering buying an additional 385. Even Saudi officials cannot explain where they will find trained drivers and crews for 700 such tanks. One Saudi official suggested that some of the tanks might be stored for use by U.S. personnel in case of another crisis.

Few Saudis have ever fired a shot in anger, seen war or have any taste for the regimentation of army life. Saudi Bedouin tribesmen have long preferred the national guard, which works more like a welfare and patronage system than a regular force. Of the 56,000 guardsmen, about half are levied from tribes, to be on call when needed or when they need the money.

"We've always had a problem recruiting people into the army because the Saudis generally don't like wearing uniforms. We don't like being organized. We're better improvisers than organizers," remarked Prince Abdullah bin Faisal, secretary general of the Royal Commission for Jubail and Yanbu, the kingdom's main industrial expansion project.

In the current crisis, the government has had little problem getting Saudis to respond to its call for army, national guard and civil defense volunteers. Visiting journalists have seen hundreds reporting to recruitment centers here in the Eastern Province and the government has said more than 10,000 signed up in the first week of the crisis.

Army and national guard units, often seriously depleted by absenteeism, reportedly are back up to near normal strength.

At this military city 15 miles south of Dammam, Western reporters were given a chance to see one aspect of the emergency military mobilization now underway.

Starting at 3 p.m., 500 Saudi men ranging in age from 14 to over 70 assembled on the parade ground after coming from their homes and jobs.

The commanding officer said they would go through a crash course of daily two-hour drills for a month instead of the normal six-month training period. "They go back to their jobs and normal life," he explained. "Then, when the need is there, we call them up."

The idea, he and other officers said, was to use these volunteers in rear-echelon positions and for guard duty, freeing regular army soldiers to go to the front line.

Among the "instant soldiers" was Mohammed Abdullah Mubarak, who said he was "over 70" and looked it with his flowing white beard and white eyelashes. He said he had signed up "to defend my religion first, then my people and my land," and that while Saddam had once been his "hero, now he's stabbing us in the back."

An officer assured reporters that Mubarak was there not because the army intended to use such old men but because he had threatened "to go to the king" if they refused to let him in.

At the other extreme was a baby-faced 14-year-old, Louaye Akass, who said in fluent English that he had signed up out of "duty to my country." His father, who works for the state-run Saudi Aramco oil company in nearby Dhahran, thought it was "the best thing I ever did," he said.

Most volunteers were men like Mohammed Ali Algandi, 39, who works for the civilian maritime authority in Dammam and admitted he "never saw a gun before." Another was Mohammed Arfaj, 30, a computer systems analyst and 1986 graduate of Arkansas State University, who said he had signed up because he had "heard from the king there was a need for men."

Asked what he thought about the large number of American forces coming into the kingdom, Arfaj replied in a soft voice, "I wish there was no need for them."