The White House today said US-led troops will invade Iraq to hunt for weapons of mass destruction even if Saddam and his sons bowed to the 48-hour for Saddam Hussein to leave or face attack.
The administration also claimed it had the support of 45 nations to take action against Iraq but still faced widespread international opposition.
Mr Bush, vice-president Mr Dick Cheney, Defense Secretary Mr Donald Rumsfeld and Gen Richard Myers, the chairman of the Joint Chiefs of Staff, along with other top aides, held a meeting to prepare for "a possible war", White House spokesman Mr Ari Fleischer said.
"If Saddam were to leave, American forces, coalition forces, would still enter Iraq," Mr Fleischer added.
Mr Bush has set a deadline of 1 a.m. on Thursday for Saddam and his sons to leave Iraq or face an invasion.
"Their refusal to do so will result in military conflict commenced at a time of our choosing," he said in a solemn nationwide address last night.
About 300,000 US and British troops gathered around Iraq waiting for the order to launch a military assault on Baghdad.
Australia has said its forces will take part in any military operation, and Denmark could also contribute.
Secretary of State Mr Colin Powell said 45 nations backed the US in the coalition that may soon go to war. Mr Powell said 30 countries - which have offered troops, support, overflight rights and logistical assistance - were willing to be named publicly, while 15 wanted to remain anonymous at present.
Mr Bush was to lobby potential allies and other leaders ahead of war. The Kremlin said Mr Bush spoke to Russian President Mr Vladimir Putin, who has firmly opposed military action.
The White House said Mr Bush also spoke to Chinese President Mr Hu Jintao, who has called for a political solution. France's President Mr Jacques Chirac, for his part, has said that a US-led unilateral war is unjustified.
The US media and some opposition Democrats have also questioned Bush's diplomatic strategy in starting war without UN backing.
But the US administration remained unbowed. "Iraq has made a series of mistakes, including arming themselves with weapons of mass destruction, that have brought this crisis upon itself," Mr Fleischer told reporters.
US authorities stepped up their terrorist attack alert as Bush gave his ultimatum yesterday.