There were also splits within the allied countries. The president faced criticism from Democrats, while his close ally, British Prime Minister Tony Blair, tried to stem a revolt within his own party.
Australian Prime Minister John Howard said his government would commit 2,000 troops to a U.S.-led attack. "I believe very strongly the position the government has taken is right," Howard said.
But German Chancellor Gerhard Schroeder disagreed. "Does the threat posed by the Iraqi dictator justify a war, which is sure to kill thousands of innocent children, women and men? My answer in this case was and is: No," Schroeder said.
Mr. Bush issued his ultimatum after the U.S., Britain and Spain withdrew from consideration a Security Council resolution that would have authorized force. They blamed France for threatening to veto the measure, but France contended that the proposal lacked support on the 15-member council.
French President Jacques Chirac said a war without the support of the United Nations would undermine future efforts at peaceful disarmament.
"To act without the legitimacy of the United Nations, to favor the use of force over law, is taking a heavy responsibility," Chirac said.
At the Vatican, which is staunchly against war in Iraq, spokesman Joaquin Navarro-Valls said "whoever decides that all peaceful means available under international law are exhausted assumes a grave responsibility before God, his own conscience and history."
China's new premier called for "every effort" to avoid a conflict and said that U.N. weapons inspections must continue even as he acknowledged that the situation appeared dire.
Russia's lower house of parliament on Tuesday decided to indefinitely put off a vote on ratification of a U.S.-Russia nuclear arms treaty because of the U.S. threat of war. The treaty calls on both nations to cut their strategic nuclear arsenals by about two-thirds, to 1,700 to 2,200 deployed warheads, by 2012.
In Indonesia, a government spokesman lamented the apparent breakdown of diplomacy. "We still believe that a solution to the crisis should be found within the U.N. Security Council," spokesman Marty Natalegawa said.
The countdown to war has raised the prospect of a backlash by Muslims in Indonesia and Malaysia, making it difficult to crack down on radical groups.
In New Zealand, Prime Minister Helen Clark said it was "highly debatable" whether a U.S.-led strike on Iraq would be justified under international law.
India issued a veiled criticism of U.S. unilateralism, while Pakistan planned an emergency session of Parliament on Wednesday to discuss Iraq.
Japan supported the U.S. position. "It was a decision that had to be made," Prime Minister Junichiro Koizumi said.
In Tokyo, Koizumi described the ultimatum as "a very difficult decision" for Mr. Bush and reiterated his government's position that there was no need for a new U.N. resolution authorizing an attack.
Japan's constitution bars its armed forces from fighting in foreign wars, but Koizumi's government reportedly was considering humanitarian missions.
South Korea previously has said it supports U.S. efforts to fight the spread of weapons of mass destruction and has indicated it may send military engineers to help assist U.S. troops in the war.
In Mexico City, President Vicente Fox said that he regrets that the conflict appears headed for war, but that his nation's opposition to military action would not strain relations with the United States.
Mexico, a member of the U.N. Security Council, had struggled with its position on Iraq, as Fox walked a fine line between offending voters at home who overwhelmingly oppose war and antagonizing the United States, which accounts for about 75 percent of Mexico's trade.
"We maintain our belief that the diplomatic means to achieving (the goal) have not been exhausted," Fox said.
In Israel — where people have been instructed to build sealed rooms for protection against a possible Iraqi attack with chemical or biological weapons — Jerusalem resident Eliyahu Ben Haim backed the president.
"I think it's only 12 years too late, but yes, I think he needs to be dealt with. This is a monster," he told CBS News Correspondent Robert Berger.
In Washington, Senate Democratic leader Tom Daschle charged that a collapse of the administration's diplomatic efforts had brought an unneeded war.
"I'm saddened, saddened that this president failed so miserably at diplomacy that we're now forced to war," Daschle said in a speech to the American Federation of State, County and Municipal Employees. "Saddened that we have to give up one life because this president couldn't create the kind of diplomatic effort that was so critical for our country."
However, there were some signs of the traditional backing that the president enjoys from lawmakers, even those of the opposing party, during wartime. That even extended to funding the war's cost, which the White House had yet to formally estimate.
"I will support whatever appropriations are necessary for the safety of our troops who have been sent into harm's way, even though I do not agree with the policy of pre-emption that is sending them to Iraq without the support or endorsement of the United Nations," said Sen. Robert Byrd of West Virginia, top Democrat on the Senate Appropriations Committee.