President Bush decided on Aug. 6 to launch Operation Desert Shield despite the lack of a detailed war plan for fighting Iraq or a strong initial recommendation from his military advisers to commit U.S. forces to Saudi Arabia, according to well-placed sources.

Gen. Colin L. Powell, chairman of the Joint Chiefs of Staff (JCS), and other senior military officers did not oppose the operation. But they cautioned Bush that the military risks were enormous, including the prospect that the first U.S. troops dispatched to Saudi Arabia would be extremely vulnerable to Iraqi attack. If he was going to send troops to the Middle East, Powell told the president, it should be done on a massive scale in accordance with a secret contingency plan, the U.S. Central Command's Operations Plan 90-1002.

"Ten-oh-two," as the Joint Chiefs refer to the document, is a modified version of a plan first drafted in the 1980s that envisioned fighting the Soviet Union or Iran -- not Iraq -- in the Persian Gulf. Now the guiding document behind Desert Shield, it called for a huge air and sealift, as well as deployment of heavy armor and antitank forces on the ground. It also contains a blueprint for U.S. offensive operations.

Without that full commitment, Powell advised, no forces should be sent, the sources said.

On the 20th day of Desert Shield, following an unprecedented effort to move men, machines and supplies to the region, the U.S. military leadership now feels confident of undisputed air and sea superiority in the gulf. The imminent arrival of the 24th Infantry Division (Mechanized) and M-1 tanks will end the period of naked vulnerability on the ground. Deployment of a sophisticated network of computers and satellites also has given U.S. forces an immense advantage over Iraq in intelligence gathering.

But during the initial hours after Iraq's invasion of Kuwait, senior military officers, including Powell, could see no viable military options. Although the U.S. military had closely watched the massing of Iraqi troops on the Kuwait border, the invasion had come as an unpleasant surprise to Powell and the four other members of the JCS, and caught the military unprepared for a confrontation.

"The first day the military came in and said, 'I don't know what we can do. We've got no guys on the ground,' " a senior administration official said yesterday.

Even with 90-1002 on the table, the invasion was followed by a long period of uncertainty among military leaders while they waited for the president to decide whether the United States would immediately try to liberate Kuwait, take reprisals against Iraq, defend Saudi Arabia or do nothing. "I don't know that we were reluctant," one of the chiefs said Friday, "but I do know that we were concerned that first of all we needed a military mission." That mission was five days in coming. Unlike Bush's quick decision to invade Panama last December -- made with the forceful and unanimous recommendation of Powell and other senior military officers only 19 hours after the murder of a U.S. serviceman -- the president pondered his course in the Middle East for 115 hours. During that period -- from 9 p.m. Wednesday, Aug. 1, when the invasion of Kuwait was detected by U.S. intelligence, until 4 p.m. on Monday, Aug. 6, when Bush ordered troops to the gulf -- the JCS wrestled with their plans and the consequences of military action.

They contemplated the daunting logistical obstacles in moving tens or hundreds of thousands of troops to a region where the United States had no military bases, and the even more arduous task of sustaining those forces. They worried about the harsh desert and the prospect of fighting in 100-plus degree heat, about Iraq's demonstrated willingness to use chemical weapons and about the possibility that the first U.S. troops to arrive might be slaughtered before U.S. tanks and other heavy weaponry could arrive to protect them.

The chiefs also considered that a war or protracted standoff could be undermined by waning political support at home, a concern that still weighs heavily on the JCS. "All the chiefs understand that the United States is an impatient country," one four-star general said. "The big question," another four-star added, "is the political will question."

Heeding Powell's advice, Bush considered no military options other than the full deployment outlined in 90-1002. Once his decision was made, Bush "was the hawk," one senior U.S. official said. "The heart of the oak in all of this," another adviser agreed, "was George Bush."

It was Bush who made the key decision to move. But unlike previous eras, when presidents and their civilian advisers took active roles in military operations, Powell and other senior officers have had extraordinary leeway in the unfolding gulf crisis, making virtually all the military choices. Referring to the former president and former defense secretary during the Vietnam War, one four-star yesterday said, "There is no Lyndon Johnson or Bob McNamara picking targets or deciding which ops {operations} plan to use."

Fortified by the president's resolve, the military now believes that further Iraqi aggression can and will be met with a devastating counterpunch aimed at destroying enemy forces and military facilities. "This is not gradualism," a senior general said Friday.

Buoyed by the success of the operation thus far, military and civilian officials have been willing in the past few days to talk more candidly about the confusion and deep uncertainty of the first few days of August. Watching the Buildup

Throughout late July, Powell carefully watched Iraq's buildup along the Kuwaiti border. U.S. intelligence agencies estimated during the weekend of July 21-22 that Saddam Hussein had moved 30,000 troops into the region; but the Pentagon believed an invasion was less likely than a more-modest shakedown, perhaps the seizure of a single oil field or the occupation of two small islands in the gulf to demonstrate Iraqi indignation at Kuwait's refusal to abide by Organization of Petroleum Exporting Countries oil quotas.

In just 10 months as chairman, Powell, 53, already had overseen five U.S. military crises, including Operation Just Cause in Panama. He was not initially alarmed by Iraq's bellicosity, which looked like just another bit of Mideast saber-rattling. Though not reluctant to use military power, the general is a product of the Vietnam War, and is cautious in recommending the use of force. On a small white card labeled "Colin Powell's Rules," he had collected 13 adages to live by, including this one: "Be careful what you choose. You may get it."

So far, his military advice had been consistently accepted by the president. Confronted with the situation in the Persian Gulf, he picked his way through his choices slowly.

National security adviser to former President Ronald Reagan, Powell collects information almost compulsively from the vast U.S. intelligence network and elsewhere, leaving him with a picture of the world full of potential hotspots. But after an extensive trip to the Middle East earlier in the summer, Powell had returned to Washington feeling somewhat optimistic about peace prospects in the region.

During July, Powell was preoccupied with the Pentagon's perennial budget battles and efforts to define the U.S. military force of the future. But he asked the U.S. Central Command, or Centcom, to draft a two-tiered plan that considered possible U.S. responses to Iraqi aggression; one tier would be defensive, the other retaliatory. In the Pentagon's demarcation of the world, Centcom is responsible for U.S. military interests in a 10-million-square-mile swath stretching from Kenya to Pakistan, containing 70 percent of the world's known oil reserves. Because the United States has few military bases in the region, Centcom's headquarters are at MacDill Air Force Base in Florida.

Also in the last week of July, the U.S. military conducted a previously scheduled exercise called Warrior Flag '90, testing ability of participating commanders -- including Powell -- to communicate around the globe. Although Warrior Flag was oriented toward the Mideast, it was not specifically geared toward Iraq, Kuwait or Saudi Arabia; maps were deliberately cut up and rearranged so that the fictional countries involved bore no resemblance to real nations.

By Monday, July 30, the Iraqi force threatening Kuwait had grown to 100,000. Still, no alarm bells went off at the Pentagon because Iraq had not moved up sufficient quantities of four components considered crucial to any invasion: communications, artillery, munitions and a logistics "tail" capable of supporting attacking armored forces.

By Wednesday, Aug. 1, however, all four components were in place. The Central Intelligence Agency, in an analysis presented to the White House and Pentagon, concluded that Iraq had capability and intent to invade. At mid-afternoon, Powell and the other chiefs gathered in the "Tank," the secure inner sanctum conference room adjacent to the National Military Command Center in the Pentagon to hear an assessment from Gen. H. Norman Schwarzkopf, the Centcom commander.

Schwarzkopf, a former football player who fills a doorway when he moves through it, is the kind of man who accumulates nicknames: the Bear, Stormin' Norman, H. Norman Cigar and -- inevitably for a commander -- the Old Man. The West Point yearbook of 1956 describes him as "a connoisseur of life" who was certain to succeed. A winner of three Silver Stars and two Purple Hearts in Vietnam, Schwarzkopf had later been senior ground commander during the U.S. invasion of Grenada in 1983.

In briefing his fellow four-stars, Schwarzkopf did not predict an invasion but rather gave a "status report" on Iraqi forces and outlined their possible actions, according to one chief; he also briefly recounted possible U.S. options in the Arabian Peninsula as detailed in Operations Plan 90-1002.

After the briefing, "No one said, 'This is it, god damn it, he {Saddam} is coming,' " one senior official said. Schwarzkopf returned to Florida later that day and the chiefs went about other business.

A few hours later, Iraqi troops stormed across the border and crushed Kuwait. Powell and other key officials learned of the invasion at 9 p.m. EDT, and a crisis cell of specialists and intelligence analysts worked through the night in the Pentagon's command center. Adm. David Jeremiah, JCS vice chairman, remained with them while Powell got nearly a full night's sleep -- the last he would have for some time.

As the impact of Iraq's aggression hit home on Thursday, Aug. 2, a sense of helplessness afflicted many military officers. Despite planning for dozens of possible contingencies throughout the world, despite spending more than $2 trillion to rebuild the U.S. armed forces during the past decade, the Pentagon faced a crisis for which it had no immediate remedy or redress. "There was some resignation that, okay, we're going to have to reorganize and start on this problem from a fresh viewpoint," one of the chiefs recalled last week.

As another official put it, "What the hell could we do?"

Much of the initial debate among senior government officials on Aug. 2 and 3 focused on diplomatic and economic retaliation against Iraq, and possible covert action to destabilize and topple Saddam. During an early White House meeting, Chief of Staff John H. Sununu half-seriously proposed to Defense Secretary Richard B. Cheney that the B-2 "stealth" bomber be unleashed on Iraq. "I've only got one," Cheney replied, alluding to the fact that but a single plane has been tested sufficiently to be considered close to combat-ready.

For more than a decade, the United States had contemplated possibility of military action in the Persian Gulf. After the fall of the shah of Iran in 1979, President Jimmy Carter had created a Rapid Deployment Force with a prime mission of protecting Mideast oil fields; Centcom, created in 1983, was a direct descendant of the RDF, and Op Plan 90-1002 had evolved from contingency plans drafted in the early 1980s. A Lack of Details

But the plan did not envision loss of Kuwait, or Iraq as an adversary. It lacked the kind of detail that U.S. war plans contained for potential battlefields in Korea and Central Europe, where thick "battle books" described which tanks and antiaircraft units would deploy behind particular rocks and trees. And the Pentagon did not have several months to polish and rehearse, as it had before the invasion of Panama. "We essentially started from zero," an Air Force general said last week.

Moreover, the United States had no bases in Saudi Arabia; the Saudis and other Arabs have long feared U.S. efforts to gain a toehold in the region. In considering the current crisis, U.S. officials believed they could build on the good will earned during the 1987 reflagging of Kuwaiti ships when, as one State Department official put it, "American forces showed we could come in and then remove ourselves without breaking a lot of crockery. That was the key: being able to remove ourselves."

Many of these factors were laid out for Bush in the initial military briefings, sources said. Although a brigade of 2,500 troops from the 82nd Airborne Division could reach the gulf overnight, any sizeable U.S. force would take four weeks. Even then the United States could not possibly gain numerical superiority of Saddam's million-man army and 5,500 tanks.

Bush asked that a full presentation of military options be delivered to him at Camp David on Saturday, Aug. 4. Cheney was told that he would be sent to Saudi Arabia to present details to King Fahd on the U.S. military deployments that could be made to protect his country from an Iraqi invasion.

Over that weekend in the Pentagon, sources said, many senior officers found themselves frustrated by the delay in receiving an order specifically defining their mission. The Cheney mission was on, then off, and on and off again before he was finally sent on Sunday, Aug. 5.

Meanwhile, Schwarzkopf's staff was working furiously to adapt Plan 90-1002 for the defense of Saudi Arabia. The Time Phase Force Deployment (TPFD), a computer-generated plan spelling out what forces and equipment go when and by what means, was updated to coordinate communications, housing, water, air defense and other needs peculiar to the Saudi desert.

Cheney and Schwarzkopf were then able to present Fahd with time lines showing how deployments would be phased over many weeks, as part of a gradual but massive buildup.

Powell, having served as national security adviser, is a devout believer in the necessity of political underpinnings to any military operation. "There is no legitimate use of military force without a political objective," he often says. In the first two days after the invasion, he was offering so much political counsel that Cheney firmly suggested that the president would be better served if Powell offered more military advice, according to two officials. Final Recommendation

The chairman's final recommendation, delivered over the weekend, was conditional: if you decide to take military action, Mr. President, commit our forces fully and adequately. Powell based this recommendation on four conclusions:

Saddam did not want war with the United States. Ruthless but not irrational, the Iraqi leader knew that he would lose a full-scale shootout with the American superpower.

If military action was taken, at least a token U.S. force had to get to Saudi Arabia immediately as a resolute demonstration of U.S. commitment to defend the kingdom.

Any U.S. deployment should be clearly visible. Saddam must realize that to attack the Saudis was to attack the Americans.

Sufficient force had to be sent under Plan 90-1002 to guarantee control of air and sea. Ground forces had to be sufficient to muster a credible deterrent and fighting force.

"Once we did something," one of the chiefs said Friday, "we meant to do it with sufficient resources. And the political message would not be lost on Saddam Hussein that we are darned serious about this . . . . No nation has ever been well served by protracted conflict."

Waiting for the president's decision over the weekend, the military took small, preparatory steps. Special Operations units, including hostage rescue teams, moved to the Middle East. The USS Independence aircraft carrier battle group was directed to move from the Indian Ocean to the northern Arabian Sea. Extra C-5 and C-141 aircraft crews began arriving in Europe to expedite an airlift; 28 crews moved to Rhein-Main Air Base in West Germany and 26 moved to the U.S. base at Torrejon, Spain.

At 3 p.m. on Sunday, Aug. 5, Bush flew back to Washington from Camp David. Before meeting with his national security advisers, a tense and angry president vowed to reporters that the Iraqi occupation of Kuwait "will not stand."

At 4 p.m. on Aug. 6, Bush approved deployment of U.S. combat forces. He gave the Joint Chiefs three missions: deter further Iraqi aggression, defend Saudi Arabia, improve the overall defense capabilities of the Saudi peninsula. An hour later, a wing of F-15 fighters left for Saudi Arabia.

The order alerted the military to be prepared for other possible missions but said nothing about forcing the Iraqi occupiers out of Kuwait. At least some senior military leaders, having heard the president make that point explicitly and repeatedly on television, realized that their mission was still limited and did not yet encompass the president's ultimate stated political objective.

Staff researchers William F. Powers Jr. and Lucy Shackelford contributed to this report.