President Bush, who met with leaders of allies Britain and Spain in the Azores Islands Sunday, said today would be "a moment of truth for the world."
The leaders told the world it would get one last chance to support disarming Iraqi President Saddam Hussein, but were vague as to what the consequences would be.
In response, Saddam released a statement saying that if attacked, Iraq would take the war throughout the world to "wherever there is sky, land or water."
Bush urged other nations to support "the immediate and unconditional disarmament" of the Iraqi leader. France, Germany and Russia have opposed an additional United Nations resolution to set an ultimatum for the Iraqi leader to disarm. And efforts to win the votes of uncommitted nations at the U.N. Security Council faltered in recent days.
Bush, along with leaders of Britain, Spain and Portugal, met at an American air base in the Azores, Portuguese territory in the Atlantic Ocean.
They gathered with more than 250,000 troops gathered in the Persian Gulf area poised to strike if and when the president gave the word.
"Now we make a final appeal to make a strong, unified message on behalf of the international community," said British Prime Minister Tony Blair.
Spanish Prime Minister Jose Maria Aznar said the agreement among the leaders marked "a last chance, one last attempt to reach the greatest possible consensus among ourselves."
Bush sounded like he didn't expect reluctant countries to change their minds.
Asked whether today was the day that would determine whether diplomacy could work, he replied, "That's what I'm saying."
The Associated Press contributed to this report.