There are several, but the original justification for going to war was that he possessed Weapons of Mass Destruction (WMD's), and might use them on our friends in the region, like Israel, or give them to terrorists like bin Laden.
After a thorough, year and a half long search, we found that he had none, and under long term questioning by a guard in his prison cell, Hussein admitted that he had destroyed them all after the 1991 war.
Other justifications since then have been:
1) to remove a brutal dictator from power
2) to establish a democracy in Iraq
3) to remove al-Qaeda from the country (they were not present until after we invaded)