Why did the US invade Iraq in 2003?

Answer 1

The US attacked Iraq in 2003 based on either misformation or disinformation, and the stated reasons have changed over time.

The Initial Justification : Weapons of Mass Destruction
The primary 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--neither of which had any connection to Iraq). It was suggested that Hussein might provide these weapons to terrorists to attack the US. He had already used chemical weapons sold to him by the US against Iran, and also against Iraqi Kurds. Before the war UN inspectors announced they had found no evidence of WMDs, despite unfettered access. After the war was under way, additional investigation concluded Saddam did not have WMDs.

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.

It was suggested early on the conflict was predominantly an attempt by the US to control the flow of oil from Iraq, one of the largest petroleum producers in the Middle East.

Secondary Justification: 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. The goal of the 1991 operation was to drive Saddam from Kuwait, at which the US and its allies succeeded. When no WMDs were found, the US asserted 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.

Answer 2

The US believed that Iraq had weapons of mass destruction.