The White House says the diplomatic window has closed
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This is the biggest gamble of George W Bush's already eventful presidency.
In the words of Mr Bush, it is the security of the world that is at stake - and the future of democracy in the Middle East.
"Instead of drifting along toward
tragedy, we will set a course toward safety," he said in a televised address to the nation on Monday night.
Mr Bush was resolute and calm as he outlined the threat Saddam Hussein posed to the United States and its allies, and said that Americans would not be intimidated by thugs.
"Many nations do have the fortitude and resolve to act against this threat to peace," he added.
But it is also the future of Mr Bush's own party that is on the firing line.
Domestically, success against Iraq would almost certainly ensure his re-election to the White House and domination of US politics for years to come.
But failure would surely mean the end of his political career, with public opinion polls showing most Americans supportive of war but unconvinced it is really necessary.
Failure would also mean that issues like taxes and the state of the economy would come to the fore, where the president is on less sure ground and his own party is divided.
Internationally, the stakes are, if anything, higher.
Defining moment
This is a defining moment in world history that some have compared to the start of the Cold War in the late 1940s.
The administration hopes that victory in Iraq would spread democracy in the Middle East, deter other dictators and vindicate its policy of pre-emptive action.
Mr Bush was very explicit about the need for regime change in Baghdad, and appealed directly to the Iraqi people in his speech, saying the US would liberate them from an oppressive dictator.
The scenario, if realised, would strengthen America's world dominance.
And, in that view, it could usher in an American century where liberty and freedom are the central theme.
By the same token, a long drawn-out war threatens to undo America's decades-long alliance with western Europe, destabilise the Middle East and undermine confidence in the US economy.
As war approaches, President Bush himself is said to be serene - a serenity aides say comes from his conviction that what he is doing is right, despite the worldwide chorus of doubt and disapproval.