President Bush vowed last night to attack Iraq with the "full force and might" of the U.S. military if Saddam Hussein does not flee within 48 hours, setting the nation on an almost certain course to war and denouncing countries that refuse to support him.

Bush delivered the ultimatum hours after his administration earlier in the day admitted failure in its months-long effort to win the blessing of the U.N. Security Council to disarm the Iraqi leader by force. The United Nations ordered its inspectors and humanitarian personnel out of Iraq, and Bush urged foreign nationals to leave the country immediately.

"The tyrant will soon be gone; the day of your liberation is near," Bush said in remarks intended to be translated for broadcast to Iraqis. Promising to rebuild an Iraq without oppression, he urged Iraqis to lay down their arms.

"It is too late for Saddam Hussein to remain in power," he said. "It is not too late for the Iraqi military to act with honor and protect your country, by permitting the peaceful entry of coalition forces to eliminate weapons of mass destruction."

The ultimatum triggered a wave of offensive and defensive measures across the world. Bush declared a state of "heightened watch" for terrorist strikes in the United States. Vowing to fortify borders and patrols, U.S. officials raised the national terrorism alert to "code orange," the second-highest level of danger.

In the Persian Gulf region, about 300,000 U.S. and British troops prepared for action, while on Wall Street stocks rallied as the prewar uncertainty ended. In Britain, a senior member of Prime Minister Tony Blair's Cabinet resigned in protest of the coming war while Parliament readied an antiwar vote. The likelihood of imminent hostilities also led government leaders in Turkey to revisit the legislature's earlier refusal to allow U.S. troops to use Turkish soil for an attack.

Bush's nationally televised speech, from the Cross Hall in the White House, was tantamount to a war announcement in the view of administration officials, because Iraq has made clear that Hussein will not accept exile. While the president said hostilities will begin "at a time of our choosing," a senior official said last night that Bush is likely to make a second speech to the nation after the ultimatum expires on Wednesday night, announcing an attack.

Bush presented grim images of the potential danger of terrorist strikes on U.S. soil.

"We choose to meet that threat now, where it arises, before it can appear suddenly in our skies and cities," he said. He spoke darkly of acting "before the day of horror can come."

The address included a recitation of the now-familiar history of U.S. and U.N. efforts to disarm Hussein since the Gulf War in 1991, and the dossier of the Iraqi leader's weapons programs and terrorist ties. But, with the completion of the administration's diplomatic debacle at the United Nations, he added to his remarks a sharp condemnation of what he views as a feckless Security Council and the leading holdout, France.

"Some permanent members of the Security Council have publicly announced they will veto any resolution that compels the disarmament of Iraq," Bush said. "These governments share our assessment of the danger, but not our resolve to meet it.

"Many nations, however, do have the resolve and fortitude to act against this threat to peace. And a broad coalition is now gathering to enforce the just demands of the world. The United Nations Security Council has not lived up to its responsibilities. So we will rise to ours."

Earlier in the day, British and U.S. diplomats, facing certain defeat on the Security Council, withdrew a resolution that would have cleared the way for war. Though Bush on Sunday vowed another day of "working the phones," it quickly became clear that as many of 11 of 15 council members remained opposed. The effort was abandoned by 10 a.m.

The withdrawal of the resolution without a vote was a double climb-down for Bush. On Feb. 22, he had predicted victory at the United Nations, and on March 6 he said he wanted a vote regardless of the outcome.

Last night's speech was the culmination of a case that Bush has steadily built against Baghdad, beginning with his State of the Union address in January 2002, when he branded Iraq, North Korea and Iran as an "axis of evil."

Last September, Bush called on the United Nations to confront Hussein, and two months later the Security Council unanimously approved a resolution saying Hussein would face "serious consequences" if he did not disarm. In October, Bush won votes from both houses of Congress authorizing him to use military force against Iraq.

But with U.N. weapons inspectors reporting halting progress in their work in Iraq, Security Council members, including France, Russia, China and Germany, were opposed to U.S. efforts to cut off inspections and present Iraq with an ultimatum.

Instead, Bush issued the ultimatum on his own at 8 p.m. "All the decades of deceit and cruelty have now reached an end," he announced. "Saddam Hussein and his sons must leave Iraq within 48 hours. Their refusal to do so will result in military conflict, commenced at a time of our choosing. For their own safety, all foreign nationals, including journalists and inspectors, should leave Iraq immediately."

Hussein's two sons, Uday and Qusay, both in their late thirties, are described by U.S. intelligence as ruthless successors-in-waiting who have abetted torture and enforced his police-state tactics. The administration has said both should be tried as war criminals.

Bush defiantly asserted a right to attack Iraq, even without sanction from the Security Council. "The United States of America has the sovereign authority to use force in assuring its own national security," he said. "The United States and our allies are authorized to use force in ridding Iraq of weapons of mass destruction. This is not a question of authority. It is a question of will."

Bush made frequent references to a peace-loving United States that enters the conflict with Iraq reluctantly. "The United States and other nations did nothing to deserve or invite this threat," he said. "But we will do everything to defeat it." He said "the American people can know that every measure has been taken to avoid war, and every measure will be taken to win it."

In a major passage of the speech directed at Iraqis listening on translated radio broadcasts, Bush urged soldiers to surrender to the U.S. soldiers' peaceful intentions. "I urge every member of the Iraqi military and intelligence services, if war comes, do not fight for a dying regime that is not worth your own life," he said.

Bush committed himself to providing food and medicine in the short term, and freedom in the long run.

"We will tear down the apparatus of terror and we will help you to build a new Iraq that is prosperous and free," he said. "In a free Iraq, there will be no more wars of aggression against your neighbors, no more poison factories, no more executions of dissidents, no more torture chambers and rape rooms."

In his remarks, the president also presented a concise articulation of his "preemption" doctrine allowing the United States to attack emerging threats.

"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. The security of the world requires disarming Saddam Hussein now."

At a National Security Council meeting convened at the White House at 8:55 a.m. yesterday, Bush finalized the decision to withdraw the resolution from consideration and prepared to deliver an address to the nation that had already been written.

White House press secretary Ari Fleischer said that about half an hour later, Bush told him words to the effect of, "the time has come." Fleischer announced the decision to issue the ultimatum from the White House podium at 10:10 a.m., minutes after the allies withdrew the resolution.

Rather than calling doubters, Bush spent much of the day talking to committed allies, including the prime ministers of Britain and Spain, who co-sponsored the U.N. measure; Prime Minister Simeon Saxe-Coburg-Gotha of Bulgaria, the only other council member to support it; Israeli Prime Minister Ariel Sharon; and Australian Prime Minister John Howard, who said Bush had asked him to send Australian troops to Iraq.

War has been widely considered inevitable for weeks. But the realization that nearly six months of seemingly endless U.N. discussions were finally over and the invasion about to begin unleashed a torrent of world preparation and comment, much of it disapproving.

Bush did not say what nations would be in his "coalition of the willing" that attacks Iraq. Aides said it would be "substantial," but acknowledged few nations beyond Britain and Australia have committed to supplying troops. Much of the other support will come in the form of financial and technical help with the reconstruction, as well as overflight rights and use of bases during the war.

"We've never overstated the combat role," a senior official said. "But you can't have combat if you don't have supplies, and the overflight and basing make that possible."

The White House has refused to offer official estimates of the cost of the war, and Bush provided none last night. Sources said he plans to ask Congress in coming days for about $80 billion for the combat phase and the initial reconstruction of Iraq.

The speech had been in the works for weeks. A senior official described the final trajectory of Bush's decision as "a snowball picking up speed" ever since the previous Monday, when French President Jacques Chirac said he would veto any resolution threatening war.

Aides described Bush as relaxed and at peace with his decision, and television cameras captured him -- in coat and tie -- throwing a stick and playing ball with his dogs, Spot and Barney, on the South Lawn. He met, as scheduled, with trustees of the Social Security system and held meetings with several Cabinet members.

During what an aide called the "tense days of waiting" before the ultimatum expires, Bush will remain largely out of sight.

The mood around the White House was mostly somber, with an increased security around the grounds and on neighboring streets. One senior official spoke of a sense of relief that the diplomatic wrangling was over, saying, "Finally, after so many weeks, we're moving forward."

U.S. troops in Kuwait shared that sentiment. About two hours after the president spoke, Lt. Col. Stephen Twitty told his battalion of the 3rd Infantry Division, "Guys, you're going to war."

The troops, standing in formation on a dusty patch of desert, roared back, "Hooah!"

Twitty reminded the soldiers of the 3rd Battalion, 15th Regiment, that they would be encountering many civilians.

"We're not going up there to fight the Iraqi people. The Iraqi people are good people. They've just been put in a bad situation."

He told the troops to treat Iraqi soldiers that surrendered with "dignity and respect."

Soldiers reacted to the president's speech with a sense of relief that their mission has finally been settled after more than five months in the desert of northern Kuwait.

"You feel like the end is going to come," said Chief Warrant Officer Angel Acevedo, 36, of San Juan, Puerto Rico. "We're finally going to get this over with. The soldiers are ready to do it and go home."

Staff writer William Branigin in Kuwait contributed to this report.

U.S. Marines stationed at Camp Shoup in Kuwait listen to President Bush, in a speech from White House, deliver ultimatum to Iraq.