The United States previously went to war with Saddam Hussein in 1991 to force his invading armies out of Kuwait.
At the end of the Gulf War, a truce was signed leaving Saddam Hussein in power.
The truce was sealed by the United Nations (UN) which established the conditions by which America would allow Saddam Hussein to retain his position as Iraq's president.
The UN drew up a list of requirements, known as resolutions, which required Saddam Hussein to disarm and to stop his programs to develop weapons of mass destruction (WMD).
These weapons included nuclear, biological, or chemical weapons which are able to cause widespread devastation and loss of life.