1. Because Sadam Hussein has been funding terrorist groups, including Hamas, Hezbollah, and al Quaeda.
2. Because Sadam Hussein has weapons of mass destruction which he has not accounted for, and was REQUIRED to do so by the UN in 1991.
3. Because Sadam Hussein and his regime have been providing technical expertise to said terrorist groups.
4. Because Saddam Hussein and his regime have been harboring terorists within the borders of Iraq (Abu Nidal, for example).
5. Because Saddam Hussein has not cooperated with the UN weapons inspectors, and has, in fact, made every effort to deceive them.
6. Because Saddam Hussein will use said WMD's on Israel and Europe, and will provide those WMD's to terrorists who will use them on the USA.
7. Because Saddam Hussein has brutalized his people, using torture and terror to maintain control of the country.
I take it you have not been paying attention.
Boot Hill