When a grim George Bush strode to the microphones on the White House South Lawn Sunday to call Iraqi President Saddam Hussein a liar and vow that the invasion of Kuwait "will not stand," the president did not even hint that he had already made the critical decisions to set in motion his third, his largest and his riskiest military operation.

He did so Saturday in the seclusion of Camp David, with no visible signs of anguish or uncertainty as he embarked on what his aides call the most momentous undertaking of the Bush presidency.

According to several senior administration officials, the Saturday session at the presidential retreat with senior foreign policy and military advisers was the decisive moment in a week that may prove to be the most important of Bush's term. Sitting in the rustic quiet of Aspen Lodge, Bush heard Defense Secretary Richard B. Cheney, Joint Chiefs of Staff Chairman Colin L. Powell and Gen. H. Norman Schwarzkopf, the chief of the U.S. Central Command, lay out the military options he had asked Powell to develop on Friday, less than two days after the Iraqi invasion.

An Air Force officer gave a detailed briefing on air power in the region and U.S. capabilities. A Navy officer offered an assessment of the naval situation. "By the end of Saturday," an official said, "there was a conclusion that we needed to put a defensive posture into Saudi Arabia to let Saddam know that an attack against Saudi Arabia was an attack against the U.S."

Hours earlier, sitting only a few feet from a small hallway where the trophies of Bush's invasion of Panama, his first military operation, are proudly displayed, the president had signed off on what would be his second military operation: The decision to send a small contingent of Marines into Liberia to evacuate Americans caught in the cross-fire of that country's civil war. But the incursions into Panama and Liberia paled in comparison to the decisions facing Bush as he confronted the crisis in the Persian Gulf.

"All of us knew this was not Panama, this was not Grenada, this was a deal with no known end, no predictability and everything a presidency has -- I mean peace and prosperity -- at stake," one official said.

En route back to the White House from Camp David Sunday, Bush called White House press secretary Marlin Fitzwater to ask whether he should talk to reporters when he landed at the White House. On the South Lawn, he was met on the lawn by Richard Haass, the Middle East expert on the National Security Council staff. Haass thrust a cable into Bush's hands and the president, scowling, read it. Officials said the cable contained further intelligence information on the extent of the Iraqi buildup that contradicted the public assertions by the Iraqis that they were pulling out of Kuwait, not massing on Saudi Arabia's border.

Striding to the microphones, Bush began the task of persuading the nation that Saddam was an enemy not just of Kuwait and Saudi Arabia, but of the United States. In harsh language he usually reserves for political campaigns, the president branded Saddam a liar and a threat.

"Watch and learn," he snapped when asked how the United States would respond.

Was Bush's Sunday diatribe staged? Senior officials insist not. "He did it because he felt that way," said an official. "There was no intention beforehand to assume a posture just for the impact."

By Sunday night, administration officials said, the military orders were being prepared and by Monday they were secretly issued. Meanwhile, Bush and his senior advisers engaged in one of the trademarks of his presidency -- an intricate dance of diplomacy through the capitals of the world, some of it public, much of it private, aimed at orchestrating an international response to the Iraqi invasion.

On Sunday the president met for the third time since the start of the crisis with his National Security Council team and plunged into what his aides call his "speed-dialing mode" as he began telephoning world leaders, most of whom he knows well. Treasury Secretary Nicholas F. Brady, a Bush friend for three decades, said yesterday that the personal diplomacy, "the extra time he has spent since his inauguration and before to get to know all these world leaders, has really paid off for the country in this situation."

But the phone calls Sunday also illustrated that while attention to personal diplomacy can help, it does not guarantee success. One of the calls was with Jordan's King Hussein, who publicly sided with Saddam, dashing Bush's hopes that the Arab world would join him in condemning the invasion.

Another call was to President Turgut Ozal of Turkey, a NATO ally whose cooperation in international efforts to cut off Iraq's oil would be vital. But Bush's most telling call of the crisis, made only hours after the Iraqi forces swept through Kuwait, was to Saudi King Fahd. The call helped lay the groundwork for the king's swift agreement to the unprecedented invitation for U.S. military forces to enter Saudi Arabia.

The Saudi negotiations are a case study in the private and public side of Bush diplomacy. When Bush dispatched Cheney and top aides to Saudi Arabia Sunday, it was widely reported that their mission was to gain Saudi concurrence for U.S. military forces to enter the country.

But, said a senior official, "the deal was essentially done in Bush's conversations" with the king. Cheney obliquely acknowledged that yesterday. Noting he had called Bush from the Saudi capital Monday to tell him of Fahd's approval, Cheney said Bush's "extensive consultations" with Fahd and his "commitment to provide extensive forces for the defense of the kingdom" had laid the groundwork for the Saudi agreement.

A senior official said the White House "couldn't afford" to correct the impression that existed Monday that Saudi Arabia's concurrence was still being sought. Bush, the official said, "knew he had to thread a lot of crosscurrents" in the Arab world, particularly Arab hostility to the West and the fear among Arab countries that the United States would not back up its promises. "He needed to be reassuring; that was the reason for most of the calls," an official said.

Cheney's secret order to the Pentagon Monday to set the military operation in motion occurred as the United Nations prepared to take a major step in the diplomatic phase of the administration's efforts. On Tuesday, the U.N. Security Council voted, without dissent, to impose severe economic sanctions, including a naval blockade, against Iraq. Bush's goal, an official said, was "to get the world to put a total economic squeeze" on Saddam.

An official said Bush "knew 24 hours after the invasion that the first step would be making this an international effort. By Thursday, the orders started flooding out of the Oval Office. The president had all of these diplomatic pieces in his head. The U.N. piece. The NATO piece. The Middle East piece. He was meticulous, methodical and personal."

On Tuesday, Fitzwater reported that Bush's phone calls to world leaders "are occurring at a rate of about one every hour or two." More than 20 would be made over a six-day period. One of Bush's longtime friends said the almost frenetic nature of Bush's phone calls and his personal involvement in all aspects of decision-making is the president's way of handling a crisis.

Bush "is not one who lets crisis eat away at him much" the friend said. "What he does is this intense routine of trying to find out every single thing, take up every moment in touching every known base. Then he's comfortable because he tells himself, 'I've done everything I could.' "

As the first troops began leaving their U.S. bases early Tuesday morning, Bush made another characteristic decision -- only his closest aides would know of the military action until he was prepared to announce it Wednesday morning, nearly 24 hours after the first military movements were easily spotted along the nation's East Coast.

Despite an intense internal debate in which several aides argued that Bush should tell the public what was happening Tuesday night before television did that job for him, the president demurred. As usual, aides said, Bush was "stubborn" in insisting he would not make the announcement until he thought it was the right diplomatic and military moment. He was also originally dubious about a televised address to the nation, a situation in which Bush is least comfortable.

When the cameras lit on Bush yesterday morning, his discomfort with staged Oval Office speeches was clear but so was his resolve. Later, he returned to the forum he likes far more -- the news conference. The nervous smiles and out-of-sync gestures of the speech were gone but Bush stumbled through several minor mistakes. At the end, he thanked reporters for correcting him.