Mike Pridmore at the DailyKos gives a plausible answer.
Simply put, George Bush invaded Iraq when he did because he realized his case for war was about to fall apart.
Bush had many reasons to invade, not all of them good:
- Capture and hold existing WMD assets
- Terminate WMD programs
- Put Hussein out of power
- Create Israel-friendly Arab state
- Punish Hussein for attempted assassination of Bush 41 in the early 90's
- Finish what Bush 41 started and was criticized for not finishing
- Capture and effectively own Iraqi oil
- Provide huge contracts for friends at Halliburton and Bechtel
- Generate public support and a chance at winning the 04 election
Rationales 1 - 4 are well-intentioned, the others are not. Unfortunately, the rationale for the WMD-related goals were obviously false. Bush deliberately undermined the UN weapons inspectors. On the eve of the war, the UN inspectors believed Hussein had no nuclear, chemical or biological weapons. As we now know, the UN inspectors were right and Bush was wrong.
Does that mean that Bush was just delusional before the war? Or did the lure of personal gain override his judgment?
Fact: Bush and Cheney created the Office of Special Plans (headed by Douglas Feith) in the Pentagon to cherry-pick intelligence that favored the invasion. Was this just to err on the safe side? Or a deliberate plan to perpetrate a war that was unjustified?
Bush is either incompetent, corrupt or really, really, really unlucky.