The fate of the Persian Gulf now rests in the hands of President Bush and Iraqi President Saddam Hussein. But as each man weighs the military, political, economic and human consequences of his next move, neither can know with any confidence how the endgame the two leaders set in motion will play out.

For Bush, who sees himself at a defining moment of a new world order, the challenge is whether to take a divided nation into an uncertain war or suffer the failure of not meeting the objectives he has outlined repeatedly -- unconditional withdrawal of Iraqi troops from Kuwait and the restoration of the Kuwaiti government. His presidency hangs in the balance.

For Saddam, the issue is his very survival as he considers whether to continue to test the will of the coalition aligned against him or withdraw. He appears to believe that if he calculates correctly, he will become the champion of the Arab world. If he misjudges either U.S. willingness to go to war or the ability of his forces to withstand an American offensive, he could be gone in a month or so, his nation in ruins.Five Scenarios for the Coming Days

With the hour late, there seem now to be relatively few possible scenarios for the coming days: (1) a sudden decision by Saddam to withdraw all his forces unconditionally; (2) the beginning of an Iraqi withdrawal that ultimately produces a messy diplomatic resolution; (3) a preemptive strike by Iraq against Israel; (4) a U.S. decision to delay military action for some time; (5) a U.S. decision to go to war.Total Withdrawal: Bush said Saturday that a "rapid, massive" withdrawal of Iraqi forces from Kuwait might yet avert a war, but with the failure of United Nations Secretary General Javier Perez de Cuellar's mission to Baghdad, U.S. officials appear increasingly pessimistic that Saddam intends to comply with the U.N. resolutions requiring complete and unconditional withdrawal.

Nonetheless, they have pointed to his sudden abandonment last summer of the gains he made in his eight-year war with Iran as evidence that he is capable of reversing course. Saddam could wait until tonight's deadline passes, conclude he has stared down the rest of the world and pack up to fight another day, with his military intact, his chemical, biological and nuclear weapons capabilities untouched and the coalition less likely to maintain tight economic sanctions against him. Messy Diplomacy: More likely, U.S. officials fear, is a decision by Saddam to start a phased or partial withdrawal, the so-called "nightmare scenario" under which Iraqi forces would pull out of southern and central Kuwait but continue to lay claim to the Rumalia oil field in the north and to the islands of Bubiyan and Warba off Kuwait's coast. This could set off a new flurry of diplomacy and create pressures on Bush and his allies to accept a solution that would fall short of full compliance with the U.N. resolutions. At a minimum, the beginnings of a credible withdrawal could make it impossible for Bush to launch military action. Geoffrey Kemp of the Carnegie Endowment for International Peace said he can think of nothing "that would do more to force a pause in the current momentum and possibly create a schism in the alliance" than this.

Short of actually starting a withdrawal, Saddam could conceivably pledge to withdraw in return for guarantees of an international peace conference on Arab-Israeli issues, a commitment from the West not to destroy his military, an end to sanctions or a promise for negotiations with Kuwait over territorial and economic issues.

Saddam could claim victory from any such diplomatic solution; the question is whether Bush would accept it. U.S. officials have indicated from the beginning of the crisis that if Saddam withdraws completely, they could accept some elements of face-saving for him, and U.N. officials envision negotiations to settle Iraq's grievances against Kuwait. If Saddam does not withdraw, U.S. officials say no deals.

In considering the possibility of a messy diplomatic resolution, House Armed Services Committee Chairman Les Aspin (D-Wis.) said in a recent analysis, "The critical test for the United States is whether the anti-Iraq coalition, in the wake of a diplomatic solution, will be able to hold together to contain Saddam and reduce Iraq's military leverage in the region."

In reality, the United States could end up a bystander to such a deal, publicly maintaining its insistence on not linking an Iraqi withdrawal to any concessions while Arab or European diplomats work out the details. One official familiar with the Middle East said the Saudis and other Arab states would have a hard time walking away from a messy compromise. Preemptive Strike: U.S. officials have long feared that Saddam would try to shatter the international coalition arrayed against Iraq by launching a preemptive strike against Israel. They have spent considerable time working with the Israelis to be clear on how the Israelis would respond to such a provocation and what the Arab members of the coalition -- the Egyptians, Syrians and Saudis -- would do if Israel gets drawn into the battle. But administration officials still fear that an Iraqi attack could disrupt the coalition and transform the gulf conflict into a regionwide war.

On Sunday, the Israelis rebuffed U.S. requests to refrain from retaliation if attacked. A delegation led by Deputy Secretary of State Lawrence S. Eagleburger left Jerusalem without finalizing tactical coordination between U.S. and Israeli military forces, should Israel launch a response to an Iraqi attack.

Some U.S. officials believe an Iraqi preemptive strike would signal that "Saddam thinks he is going down the tubes," as one senior policy-maker put it. He added, however, that there is little in Saddam's background to suggest he is a martyr. Rather, the official said, Saddam's first impulse is survival to pursue his larger ambitions. Waiting It Out: Bush said Saturday that if there is a war, it will come "sooner rather than later." But he could still decide to wait out the Iraqi leader for some time given the potential costs of waging war. That could increase the psychological pressure on Saddam, possibly bringing about a decision to change course.

But U.S. officials say the disadvantages of waiting are significant. "If the 15th comes and goes and {Saddam} stands there and has defied the United Nations, the Russians, the French, the British, the Security Council, you name it, and gotten away with it," said a senior administration official, "then he's the hero of the Arabs. Then he scares {Egyptian President Hosni} Mubarak to death, scares the Saudis to death, because that's attractive to the downtrodden Arab masses.

"You know," the official continued, "here's a guy who has been able to defy the damn Westerners. The coalition falls apart because from the coalition perspective, the United States . . . pushed getting this deadline {and} it has come and gone."Economics May Influence Timing

Economic considerations also may influence a U.S. decision on how long to wait. The economic sanctions are hurting not only Iraq but also such front-line states as Egypt, Turkey and Jordan, as well as countries in Eastern Europe and in the Third World. "A short, decisive war is probably, from a strictly economic point of view, better than prolonged maintenance of sanctions, with the prolonged threat of war hanging over . . . the markets," said Robert Hormats of the investment banking firm Goldman-Sachs. "But there is no guarantee the war will be short and no guarantee of no aftereffects. . . . "War: Even the most optimistic Pentagon plan for war, envisioning a short and decisive conflict, carries terrible risks for Bush. "The art here is going to be to do enough to destroy {Saddam's} offensive capability quickly and his forces, but not destroy Iraq in the process," said William Quandt of the Brookings Institution. "I'm not sure we know how to fight that war."

It is the hope of U.S. planners that the coalition would be able to control the course of the war and therefore its outcome, and they point to a variety of factors -- U.S. air superiority, open terrain, night-fighting ability -- to argue that the war would be swift and the allies certain of victory. Casualties in these optimistic scenarios range from several hundred American dead to a few thousand.

But all this is still guesswork. As Bush weighs a decision to launch a war, he must consider the possibility of a longer, bloodier conflict, resulting in tens of thousands of American deaths, the rapid erosion of domestic political support, spiraling oil prices, intensifying anti-American feelings throughout the Arab world and continuing instability in the region.

The American public has been prepared generally for victory at minimum price, and the polls offer no reliable guide of public opinion if things turn sour. Which is why Quandt said that no matter how well prepared, no matter how well briefed, "it will be a very sobering moment when Bush goes into his study and says tomorrow is the day. . . . "

Staff writers David Hoffman and R. Jeffrey Smith contributed to this report.