The answer is: destroy al-Qaeda network and other terrorist groups responsible for September 11th attacks by invading strongholds in Afghanistan and Iraq.
After 9/11, George W. Bush´s administration claimed that the perpetrators of the attack on US soil, Al Qaeda, were related to Iraq and Afghanistan and, in the case of Iraq, were in possession of weapons of mass destruction and with the capability of furthering their production of these. This last claim would eventually prove to be false whilst the first claim was, at best, misleading. Both invasions of Iraq and Afghanistan continue to this day.