President Bush addressed the nation last night under fire for appearing in a rush to war and diplomatically isolated from other big powers over how best to disarm Iraqi President Saddam Hussein.

His prime-time news conference was therefore a tricky balancing act, one part aimed at reassuring critics that he is not hungry to unleash the U.S. military in the Persian Gulf, another aimed at resetting the world debate in a way that puts the onus back on Hussein and the United Nations, rather than on himself.

Whether that will bring him success in a showdown at the U.N. Security Council over a second resolution, giving approval for going to war, will not be known until sometime next week, but in understated language, the president made clear that diplomatic victory is not his real objective. Instead, it is the disarmament of Hussein, voluntarily or by force, with the choice, he said, the Iraqi leader's alone.

The president offered no new arguments and no new evidence last night to buttress his case against the Iraqi leader. Instead, he put his own credibility on the line in terms designed to reassure Americans that he is not acting impulsively but with the security interests of a nation forever changed by the terrorist attacks of Sept. 11, 2001, foremost in his mind.

Through 40 minutes of questioning, Bush repeatedly returned to a set of simple arguments. Hussein, he said, is "a direct threat" to the security of the United States and that if he were willing to disarm, he would have done so already. Instead, he said, Hussein is engaged in a "willful charade" that will continue until the world forces him to change.

"I will not leave the American people at the mercy of the Iraqi dictator and his weapons," he said.

The administration faces another crucial day at the United Nations today, with chief U.N. weapons inspector Hans Blix scheduled to present what could be his final report on the progress he and his team have made in getting Iraq to comply with the demand for disarmament contained in a resolution approved unanimously last fall.

Blix's last report put the administration on the defensive and provided ammunition to those nations who say the inspectors should be given more time to force compliance without military action. Last night, Bush challenged Blix not to provide an inventory of Iraq's actions but to "answer a single question: Has the Iraqi regime fully and unconditionally disarmed, as required by Resolution 1441, or has it not?"

Bush said the answer is clearly no, but he and his advisers understand that the steps Iraq has taken recently to destroy some of its missiles have emboldened opponents of the United States to press for more time for the inspectors before setting any deadline on Iraq's compliance.

With roughly a quarter of a million U.S. troops now positioned in and around the Persian Gulf, Bush appeared determined to push the U.N. debate to a quick conclusion and move to the next phase of the confrontation.

What Bush set out to do last night was to preempt that U.N. debate by declaring that the United States will not allow the issue to drag on indefinitely. "It is now time for this issue to come to a head at the Security Council, and it will," he said. "As far as ultimatums and all the speculation about what may or may not happen after next week, we'll just see."

In tone and style, the president was somber and subdued in what appeared to be a deliberate attempt to defuse the perception, particularly abroad, that he is a cowboy determined to throw his weight around, regardless of world opinion. Bush repeatedly said he hopes war can be avoided, and he adopted a tone of resignation that Hussein has left him no other choice. While he is prepared to use force, Bush said, it is Hussein who will determine "whether we go to war. . . . He's the person who can make the choice between war and peace."

But he was anything but understated in conveying his own feelings, as when he spoke in the past tense about Hussein's failures to comply and the consequences of that noncompliance. "I recognize there are people who don't like war," he said. "I don't like war. I wish that Saddam Hussein had listened to the demands of the world and disarmed. That was my hope."

To those who claim that he is on a personal vendetta against Hussein that threatens stability in one of the most volatile regions in the world, Bush responded: "My job is to protect America and that's exactly what I'm going to do. People can ascribe all kinds of intentions. I swore to protect and defend the Constitution. . . . I take the threat seriously and I'll deal with the threat. I hope it can be done peacefully."

Bush allies applauded his effort to shift the terms of the debate at the United Nations. William Kristol, editor of the Weekly Standard and long a proponent of confronting Iraq, said the administration needs to make the final days of the U.N. debate "a sideshow" and to make clear "that we have reached our conclusion that Saddam hasn't disarmed, to make less hinge on Blix and France and the others" opposing the U.S. position.

That strategy involves huge risks, however, with the United States now preparing to go to war possibly without the support of a substantial part of the international community. "At this point, there's very little likelihood they can salvage the diplomacy," said Jim Steinberg, deputy national security adviser in the Clinton administration and now at the Brookings Institution, "so they need to get people to think it is irrelevant."

Bush held out some hope last night that more support will materialize before the Security Council votes next week, but he sounded even more optimistic that, if war comes, the United States will not act alone. "I think you'll see when it's all said and done, if we have to use force, a lot of nations will be with us."

That could come very soon, judging from Bush's comments last night. The debate at the United Nations could end by midweek, and Bush said he is prepared to warn U.N. personnel, international business people, journalists and others to leave Baghdad before war starts.

Only the president and his advisers know their timetable for initiating action if Iraq does not blink in the next days. And in calling for a vote at the United Nations, Bush challenged others to step forward as he was doing last night. "It's time for people to show their cards, let the world know where they stand when it comes to Saddam," he said.

In a final show of defiance, the president offered this challenge to the rest of the world. "When it comes to our security," he said, "we really don't need anybody's permission."

That may be correct, but it is a risky way for the president to begin what could be the most perilous undertaking of his presidency.

White House press secretary Ari Fleischer, left, national security adviser Condoleezza Rice and Chief of Staff Andrew H. Card Jr. listen to President Bush as he addresses the nation during a news conference in the East Room of the White House.