For Saddam Hussein, for George Bush, for a million of their soldiers massed across a Middle Eastern desert, the day of reckoning is here.

War may not be inevitable -- although 9 of 10 Americans polled believe it is. But the arrival at midnight (EST) tonight of the United Nations deadline for Iraq to withdraw from Kuwait means that war will now be legally sanctioned.

Not since the Cuban missile crisis has the United States faced a moment of such wrenching anxiety. Not since August 1914 has the world witnessed such a ponderously deliberate prelude to conflagration: armies plodding forward, reserves and resources being mobilized, civilians scurrying for cover, diplomats frantically trying to cage the dogs of war.

Military conflicts have erupted 200 times in American history. Yet only nine times -- roughly once every generation -- has this nation truly gone to war. A Persian Gulf war, which could become a desperate fight of unprecedented intensity and technological might, would be the 10th.

The national mood is one of somber disquietude, rather than the righteous indignation that in the past often bound Americans together in martial pique. Despite administration efforts to demonize Saddam, there is little evidence that Americans hate Iraqis or want to destroy Iraq. Across the United States, around the globe, people appear more frightened than bellicose.

In ways small and large, the prospect of war has become part of the American landscape -- in Operation Desert Shield blood drives and ubiquitous yellow ribbons, in small town send-off parades and "no war for oil" placards, in a thousand televised cameos of pensive soldiers longing to come home.

As Secretary of State James A. Baker III observed last week, this is "a defining moment in history," perhaps a last opportunity to inventory the profound diplomatic, military, economic and political consequences that lie in the fatal hinge between war and peace.

Would repulsing Saddam by force deter future aggressors and safeguard "our jobs, our way of life, our own freedom and the freedom of friendly countries around the world," as President Bush has said?

Or, as former president Jimmy Carter asserted Friday, would war have "devastating consequences . . . for decades to come in the economic and political destabilization of the Middle East"?

Would war restore Kuwait and bring enduring peace, or would it destroy Kuwait and launch a thousand terrorists around the globe, like spores on a dark wind? Would war enhance Bush's presidency as it did that of Franklin D. Roosevelt, or wreck it as Vietnam wrecked Lyndon B. Johnson?

For five months, the Bush administration has consistently argued that nothing less than the existing world order is at stake; Baker warned in August of "a new dark age" if Iraq's aggression goes unpunished. Congress, in effect, narrowly concurred by voting on Saturday to authorize combat. The confrontation with Iraq is about "aggression, oil and nukes," said Rep. Les Aspin (D-Wis.), House Armed Services Committee chairman.

In the largest terms, Bush sees the confrontation as part of a critical search for a new global equilibrium to replace the balance of terror maintained by two antagonistic superpowers for four decades. Success in the Persian Gulf could affirm the rule of law, provide a prototype for international cooperation, reinvigorate the United Nations and enhance U.S. stature as the world's preeminent military and diplomatic force. The United States is engaged in "a highly moral enterprise," former president Richard M. Nixon wrote recently, because future bullies "will take seriously U.S. warnings about aggression."

Bush also believes the existing world economic order is at stake, an order predicated on cheap hydrocarbons. Of the world's 1 trillion barrels of known oil reserves, nearly half lie beneath Iraq, Kuwait and Saudi Arabia alone; the United States alone consumes about one-quarter of the 65 million barrels pumped from the earth every day.

Many oil analysts believe war jitters could temporarily push the price above $50 a barrel -- aggravating the U.S. recession and damaging fragile economies worldwide. It is expected that such a surge in oil prices would not last long unless Saudi fields are damaged in the fighting, thus reducing otherwise plentiful supplies.

The president's aspirations for the Middle East are somewhat ambiguous, but the administration hopes that defanging Saddam would allow moderation and stability to flourish. A diminished -- though not demolished -- Iraq would remain as a counterbalance to the Moslem fundamentalism of Iran and the expansionist ambitions of Syria; by relieving Israel of the Iraqi threat, one administration official said, the Jewish state may be more receptive to an international conference that finds an enduring solution to the Palestinian conundrum.

But others see this scenario as naive and improbable. "If war comes, there will be very, very serious consequences in the Arab world," an Arab ambassador warned Friday. "Arabs have long memories. There will be a terrible backlash. The terrorists will exploit the situation and drive a wedge between the West and moderate Arabs."

Most Americans do not understand the widespread resentment and envy felt by poor Arabs toward their rich cousins in Kuwait and Saudi Arabia, the ambassador added. A recent bank study, he said, found that six gulf states together have $670 billion in assets, most of which is invested outside the Arab world; the remainder of the Arab world collectively is $208 billion in debt. "How can you expect a poor man from Egypt or Tunisia to be sympathetic to the Kuwaiti or Saudi who can spend a million dollars in the casinos in Atlantic City in one night?" the ambassador asked.

"If our objective was to preserve stability in the Middle East and the related advantages that stability provides for oil supplies from the area, then going to war will destroy those objectives," added G. Henry M. Schuler, an energy specialist at the Center for Strategic and International Studies. "We will say that the war is over when Baghdad is leveled or Saddam withdraws or whatever measure we're using. But {the Iraqis and their sympathizers} will say the war goes on and on and on. They know full well they cannot match a superpower in conventional power, so obviously the way to counter is unconventionally -- which we call terrorism."

Waging a U.S.-led war against Iraq, some analysts believe, also would imply a future role for the United States as a global policeman willing to use muscle to impose a Pax Americana on aggressors, much as Great Britain once did. Such a role always has made Americans uncomfortable and is difficult to envision with a military that is supposed to shrink by 25 percent in the next five years. Some analysts believe the cost of a gulf confict could approach the classified Pentagon estimate in the mid-1980s for a major war in Europe against the Soviet Union: $3 billion a day.

"The U.S. public doesn't want to be the world's constable," former defense secretary Harold Brown said Friday. "And people in whatever region we're operating will also resent the constable."

The consequences of war for U.S. relations with Europe and Japan are highly unpredictable. Large anti-war rallies, already tinged with anti-Americanism, have been held in Europe. Many NATO countries also fear that a large stream of "human remains pouches," as the Pentagon now calls body bags, will provoke an isolationist backlash in Congress and the U.S. public. "I'm very concerned about what American losses would mean for future American involvement in world affairs," one European ambassador said.

Certainly the immediate stakes are highest for the U.S. military. They drafted the war plan, designed the force, demanded an end to micromanagement by Congress and the White House and have spent the better part of $3 trillion from the national treasury in the past decade. They are on the line -- their lives, their fortunes, their sacred honor.

Leaders on both sides of the conflict have described with vivid belligerence the kind of war they expect. Saddam's biblical warning of "the mother of all battles," in which American troops "will swim in their own blood," echoes the "very violent, fast" combat predicted by Lt. Gen. John J. Yeosock, the senior U.S. Army commander in Saudi Arabia.

Compared to Iraq, the United States has 15 times the population, 80 times the gross national product and a globe full of allies. But in their eight-year war with Iran, the Iraqis fought capably on defense when morale was bolstered by fighting on native ground. Even the highest authorities in the Pentagon are uncertain whether the 19th province of Iraq -- as Baghdad now calls Kuwait -- would elicit the same tenacity if war erupts.

Little combat experience can be found in the U.S. armed forces below the level of lieutenant colonel. The military will fight with troops, weapons, doctrine and a force structure -- the interweaving of active and reserve units -- never truly tested in modern warfare. Whatever happens in combat, former defense undersecretary Robert Costello said Sunday, there likely will be "an agonizing reappraisal of our pursuit of technology and high performance weapons."

Politically, a gulf war is the stuff of which presidencies are made or broken. "If there's a quick success, it's all to the president's advantage. If it's long and drawn out, with consequences that are disastrous, then the president's party suffers," said Edward J. Rollins, who served as President Ronald Reagan's political director. "Whether the Democrats can benefit from that because they're pushing sanctions or a wait-and-see approach, I don't know."

Although last week's congressional debate was often eloquent precisely because it was free of partisan rancor, no politician doubts that the gulf war already looms as one of the salient issues in the 1992 elections. "Those who are put into the most awkward position are those contemplating presidential runs . . . . It's easy to be on the wrong side of whatever happens," said Mark Peterson, a Harvard University government professor.

Before the first shot is fired, Congress is already divided -- politicized -- in ways that did not occur until several years into the Vietnam War. In a sense, Congress has already held the Persian Gulf equivalent of the Fulbright hearings, the Senate inquiry into the strategic reasoning behind Vietnam which did not begin until 18 months after the 1964 Gulf of Tonkin resolution. Whether that affects Bush's legislative agenda again depends on how the war goes.

In the long run, some students of history believe this "defining moment in history" may be less momentous than it now appears. "What happens here is important," former defense secretary Brown said. "But from the perspective of 10 years from now, will it be more important than what happens in Eastern Europe or what happens with the U.S.-Japan relationship? I'm not sure that it is."

Staff researcher Lucy Shackelford contributed to this report.