And after the 11 September attacks, the US said they would think about going to war with countries involved in terrorism. The US believe that includes Iraq.
They think Saddam is making dangerous chemical weapons and they don't trust him not to use them.
In the past, Saddam made it really difficult for United Nations (UN) weapons inspectors to search Iraq. In the end they decided to leave.
But the British MP who deals with the Middle East said war could be avoided if Saddam decided to let weapons inspectors into his country.