Answer:
The Initial Justification : Weapons of Mass Destruction
The major reason given by the administration of George W. Bush was that Saddam Hussein, the leader of Iraq, was engaged in the production or acquisition of weapons of mass destruction (WMDs). These are nuclear, chemical, or biological weapons (examples are the anthrax letter attacks in the US and the Sarin gas attacks in Tokyo). It was suggested that Hussein, who was defeated by a US-led coalition in the Gulf War, might provide these weapons to terrorists to attack the US. He had already used chemical weapons against Iraqi Kurds. After the war was under way, a full investigation showed that Saddam had no WMDs, for which UN inspectors had searched before and during the war.
The war continued, however, because the occupation forces of the US were attacked by Iraqi guerrillas, many ironically supported by Iran, Iraq's enemy in the Iran-Iraq war (1980-1988). The US supported a new democratic government within Iraq, which was opposed by various Islamic groups that traditionally held power in the country. Other factions sought to aggrandize their share of the power to be had in the new political system.
The suggestion was made early on that the US was fighting to control the flow of oil from Iraq, one of the largest petroleum producers in the Middle East.
Fostering a Democracy in Iraq
Initially when the US commenced Operation Iraqi Freedom in 2003, it was a war against the regime in Iraq; to remove Saddam from power, as the US had failed to do in 1991. This was a result favorable to the Kurds of northern Iraq, who had been repeatedly attacked by Saddam during his rule. When no WMDs were found, the US claimed that its mission in Iraq was two-fold: to create a modern democracy and to stop the persistent genocides that took place in that country. The US enfranchised the Shiite Majority, provided for Kurdish autonomy in the North, and assembled a Constitution for the entire Iraqi population. This is in stark contrast to Saddam Hussein who used chemical weapons on his own population and was responsible for committing genocide against the Shiite Arabs, Marsh Arabs, Kurds, Jews, Azeri, Assyrians, Yazidi, Bahai'i, and Chaldeans among others.