WASHINGTON — President Bush delivered a final ultimatum Monday night to Iraqi leader Saddam Hussein: exile or war — decide by Wednesday night.

"Saddam Hussein and his sons must leave Iraq within 48 hours," Bush said during a prime-time address to the nation. "Their refusal to do so will result in military conflict, commenced at a time of our choosing."

Iraqi officials said Saddam would not leave. "He will stay in place like a solid rock," Iraqi Information Minister Mohammed Saeed al-Sahhaf told the al Jazeera television service.

As Bush spoke, federal authorities elevated the nation's domestic terror threat level to high (orange) from elevated (yellow) and announced an enhanced security program called Operation Liberty Shield.

Without providing many details, authorities said they were tightening security at borders, airports, seaports and elsewhere. Homeland Security Secretary Tom Ridge urged governors to deploy National Guard units to high-risk locations.

The stark developments came just hours after the administration withdrew its pending U.N. resolution, suffering a stunning diplomatic defeat, and the United Nations suspended weapons inspections in Iraq.

"The United Nations Security Council has not lived up to its responsibilities, so we will rise to ours," the president said.

Bush based his case for war primarily upon the need to prevent Iraq from arming terrorists with weapons of mass destruction.

"We choose to meet that threat now, where it arises, before it can appear suddenly in our skies and cities," he said during the 13-minute speech from the White House.

"Terrorists and terror states do not reveal these threats with fair notice, in formal declarations," he said. "And responding to such enemies only after they have struck first is not self-defense, it is suicide."

Speaking directly to Iraqi soldiers and intelligence agents, Bush said: "If war comes, do not fight for a dying regime that is not worth your own life." He warned them to ignore orders to destroy Iraqi oil wells or use weapons of mass destruction.

"In any conflict, your fate will depend on your actions," the president said. "War crimes will be prosecuted, war criminals will be punished and it will be no defense to say, `I was just following orders.'"

And, as he has in the past, he said U.S. efforts to oust Saddam were in the best long-term interests of the Iraqi people.

"The tyrant will be gone," he said. "Your day of liberation is near."

And so, the war that the world has expected and dreaded for months rushed closer:

¶ The United States, Britain and Spain — facing certain defeat on the Security Council — announced they were killing their own U.N. resolution, which would have set a deadline for full Iraqi disarmament and authorized war.

"Obviously, we seem to be at the end of the road here," said U.N. Secretary-General Kofi Annan.

¶ In Iraq, all 156 U.N. weapons inspectors and support workers searching for nuclear, biological and chemical weapons were ordered to cross borders to safety. Humanitarian aid workers also were told to leave.

¶ In the Persian Gulf region, top U.S. and British military commanders fine-tuned strategy. Unit leaders — fearing an early strike by Iraq — placed troops on higher alert. Soldiers at many U.S. bases in the region began carrying full chemical-protection gear. About 300,000 U.S. and British troops are massed in the area.

¶ Australian Prime Minister John Howard, responding to a request from Bush, on Tuesday authorized Australian troops to join an invasion. Australia already has warships, fighter jets and at least 2,000 troops in the region. Poland promised to send up to 200 soldiers.

¶ The FBI was preparing to shift thousands of agents to counter-terrorism duties and to interview thousands of Iraqi immigrants.

¶ In New York City, officials disclosed a security plan called "Operation Atlas" that would boost police patrols on ferries, subways and other forms of mass transit just after bombing begins in Iraq. In Washington, D.C., surveillance cameras and other special security measures were in place.

¶ In Washington, Senate Democratic Leader Tom Daschle of South Dakota said Bush "failed so miserably at diplomacy that we're now forced to war."

After Bush spoke, Sen. John Kerry, D-Mass., a leading candidate for his party's presidential nomination next year, issued a statement that read in part:

"Even having botched the diplomacy, it is the duty of any President, in the final analysis, to defend this nation and dispel the security threats — threats both immediate and longer term — against it.

"Saddam Hussein has brought military action upon himself … ."

¶ Aboard the aircraft carrier USS Harry S. Truman in the Mediterranean Sea, sailors watched Bush's speech on television.

Ensign Kyra Rouggly, 26, of Medford, Ore., said the crew was "anxious" with all the uncertainty and waiting.

"If we've got to do a job, let's go over and do it," she said.

¶ In Britain, a senior member of Prime Minister Tony Blair's cabinet resigned in protest over the prospect of war. "In principle, I believe it is wrong to embark on military action without broad international support," said Robin Cook, a former foreign secretary who had been serving as the government's leader in the House of Commons.

Members of Blair's Labor Party made plans for an antiwar vote in Parliament on Tuesday, as protestors briefly blocked the street near Blair's official residence at No. 10 Downing Street.

¶ In Turkey, government leaders — noting the apparent inevitability of war — said they now were ready to end weeks of delay and might vote swiftly to allow 62,000 U.S. troops to use Turkish soil as a staging area for a northern invasion of Iraq.

¶ In Israel, citizens stocked up on gas masks, bottled water, tape, plastic wrap and other supplies in case the government instructs them to spend the first hours of war in special rooms sealed against poisoned gas.

The looming war claimed its first casualties in Israel: An Israeli-Arab woman and two teen-age sons suffocated overnight in their home after sleeping in an improperly sealed room heated with a coal burner that sucked the oxygen out of the air.

¶ In Baghdad, shoppers lined up for gasoline and bread, and many residents taped their windows.

Germany, Pakistan, India, China, Greece and many other nations told diplomats and other citizens to leave Iraq. Many U.S. and other news organizations ordered reporters, photographers and other staffers out of Baghdad.

Mohammed Aldouri, Iraq's ambassador to the United Nations, said his country was preparing for a "war of aggression by the United States and Britain."

¶ In northern Kurdish areas of Iraq, a wholesale evacuation was under way. Thousands of Iraqi Kurds, frightened by the possibility of chemical and nerve gas attacks, jammed roads leading out of the city of Irbil.

"We certainly expect Saddam to use chemical gas on us," said Jihad Shuker. "Irbil could easily become another Halabja."

Five thousand people died in the Kurdish town of Halabja after it was gassed by Saddam's forces in 1998. Saddam no longer controls Irbil, the capital of Iraqi Kurdistan, but the city remains within reach of his mortars and missiles.

The rapid-fire series of events was triggered by the joint U.S.-British-Spanish statement at the United Nations.

"The time for diplomacy has passed," said Secretary of State Colin Powell.

"I think that's pretty clear. This matter cannot continue indefinitely."

Powell and his U.S., British and Spanish colleagues singled out France as their major diplomatic obstacle. British Foreign Minister Jack Straw pointedly blamed France for creating what he called "a sense of paralysis."

But the French maintained their firm stance against military action. Speaking of the imminent U.S.-British-led war, Foreign Minister Dominique de Villepin said his nation "regrets a decision that is not justified today and that risks serious consequences for the region and the world."

Russian president Vladimir Putin, another steadfast critic of Bush's Iraq policy, spoke in similar terms. He said war would be "a mistake fraught with the gravest consequences, which may result in casualties and destabilize the international situation in general."

Canadian Prime Minister Jean Chretien said his nation would not participate in an invasion of Iraq but intends to help with post-war rebuilding.

Knight Ridder Newspapers correspondents Diego Ibarguen at the United Nations; Sandy Bauers of the Philadelphia Inquirer aboard the USS Harry Truman in the Mediterranean; Sumana Chatterjee at the Department of Homeland Security; Tim Johnson of The Miami Herald at the State Department; Shannon McCaffrey at the Justice Department; Mark McDonald in Irbil, Iraq; Sara Olkon of the Miami Herald with the Air Force in the Persian Gulf region; Carol Rosenberg in Jerusalem; Daniel Rubin in Berlin; Fawn Vrazo of the Philadelphia Inquirer in London; and Jeff Wilkinson of The (Columbia, S.C.) State in Kuwait City contributed to this report.