Although if a war does break out, they might not arrive until after it's started.
Weapons could be hidden
This has happened despite the fact that Iraqi leader Saddam Hussein has destroyed 16 banned al-Samoud II missiles in the last three days.
But American military bosses say this is not a sign that Saddam is serious about getting rid of all of his weapons of mass destruction.
They say he can still make more weapons, even though United Nations inspectors are in the country. Iraq is roughly the size of France and it's thought weapons can easily be hidden from the inspectors.
The US has added all these extra troops just a day after American and British planes raided part of southern Iraq. There are claims that six people were killed in the attack.
Lots of very important things are happening in the next 10 days, meaning war could begin on the 13 March.