The United States suspected that Iraq was developing WMDs. They feared that if Iraq was successful, they would use WMDs to threaten and attack the United States. Therefore, they used the preemptive strike of "the Bush Doctrine" to stop Saddam.
"In 2002, the new U.S. president, George W. Bush, argued that the vulnerability of the United States following the September 11 attacks of 2001, combined with Iraq’s alleged continued possession and manufacture of weapons of mass destruction (an accusation that was later proved erroneous) and its support for terrorist groups—which, according to the Bush administration, included AL-Qaeda, the perpetrators of the September 11 attacks—made disarming Iraq a renewed priority. UN Security Council Resolution 1441, passed on November 8, 2002, demanded that Iraq readmit inspectors and that it comply with all previous resolutions. Iraq appeared to comply with the resolution, but in early 2003 President Bush and British Prime Minister Tony Blair declared that Iraq was actually continuing to hinder UN inspections and that it still retained proscribed weapons."
("Iraq War (2003-11)").
After 9/11, Iraq refused to help the United States fight Al Qaeda. Saddam said that Iraq was also suffering from terrorists in the form of the economic sanctions set up by the United States ("The Iraq War Full").
Former Iraqi Secret service man, on what happened in the early stages between the US and Iraq ("The Iraq War Full").
Dick Cheney on Why We Intervened
("The Iraq War Full").