They are ready to go into the country to destroy Saddam Hussein's weapons of mass destruction after he refused to get rid of them himself.
Missiles are being made ready and huge convoys of soldiers and tanks have been moving across the desert towards Iraq.
US and British planes have fired at Iraqi arms sites but the Ministry of Defence said that's not the start of the war.
Out of time
President Bush's deadline for Saddam Hussein to leave Iraq ends at 1am UK time on Thursday.
That will be followed by soldiers invading on the ground.
17 Iraqi soldiers are said to have already surrendered to US troops on the Kuwaiti border.
US commanders are confident that they can win the war in only a few days.
But already there's been a fierce sandstorm, which has made it really difficult to see.
What about Iraq?
Many will have to spend days and nights in air-raid shelters.
So they've been hurrying to buy extra supplies of food and fuel.
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