"The tyrant will soon be gone," Bush vowed as 250,000 American troops stood poised to strike.

As Bush put the nation on war footing, he also raised the terror alert to the second-highest level, warning that terrorists may strike U.S. interests at home or abroad in response to action against Iraq.

The president told the Iraqi people, "The day of your liberation is near." He said that if Saddam does not leave, the U.S. will attack "at a time of our choosing."

Bush issued his ultimatum after U.N. allies refused to back his bid for a resolution sanctioning military force. The diplomatic defeat forced Bush to move toward war accompanied by Britain, Spain, Australia and a handful of other nations in his self-described "coalition of the willing."

Bush warned that war could lead to retaliatory strikes by terrorists on U.S. interests at home and abroad, and said he had ordered increased security at airports and along U.S. waterways.

"These attacks are not inevitable. They are, however, possible," Bush said. "We will not be intimidated by thugs and killers."

Bush spoke after deciding to raise the nation's terrorism alert from yellow to orange, the second-highest category of risk.

For the first time since he drew the nation's attention to Iraq last fall, Bush focused on the questions most asked by Americans: Why war? And why now?

Spelling out the threat, he said Saddam has weapons of mass destruction he might share with terrorists, has a history of hating America and is a destabilizing force in the Middle East.

"The United States did nothing to deserve or invite this threat, but we will do everything to defeat it. Instead of drifting along toward tragedy, we will set a course toward safety," the president said from the White House.

"The tyrant will soon be gone," he said.

He expressed disappointment that the United Nations had failed to stand beside the United States.

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

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

An intense White House debate over whether to establish a timetable was settled hours before the president's speech. Some argued that Bush should not set a deadline because Saddam could use the notice to build opposition to the president's case or even launch a pre-emptive strike against U.S. interests.

Bush said that after 12 years of diplomacy and weapons inspections, "Our good faith has not been returned. The Iraqi regime has used diplomacy as a ploy to gain time and advantage."

"We are not dealing with peaceful men," he said.

Bush also addressed Iraqi troops.

"If war comes, do not fight for a dying regime that is not worth your own life," Bush said. He told soldiers to listen carefully to his warning that they should not destroy oil wells or use weapons of mass destruction.

To civilians in Iraq he said, "If we must begin a military campaign it will be directed to lawless men who direct your country and not at you."

He pledged the United States would provide food, medicine and other assistance as Iraq recovers from war.

The address came 24 hours after Bush's return from an Atlantic island summit, where he joined with allies from Britain and Spain to give the U.N. Security Council one day consent to disarming Saddam with force.

A quick round of telephone calls Sunday night and Monday morning confirmed what aides said Bush had concluded before the summit: The allies' U.N. resolution was doomed to fail.

He ordered the measure withdrawn to avoid an embarrassing defeat, then gave the go-ahead for a long-planned ultimatum address.

The American public, by a 2-1 margin, generally supports military action against Iraq to remove Saddam, a slight increase from recent weeks, according to a CNN-USA Today-Gallup poll out Monday. Opinion was evenly divided when people were asked about an attack without an attempt to gain U.N. backing.