Here are all the reasons I can think up for why American war planners wanted to invade Iraq:
* They really believed Iraq had WMD: None of the insiders could have believed this. More and more US and British officials have exposed just how little evidence there was - and how much counter-evidence. Even George Tenet has said that his "slam dunk" comment only referred to how easy it would be to convince Americans that there were WMD. Insiders knew that Sadaam had not had WMD since 1991, and that UN sanctions since then had crippled him. He was no threat. If anything, Iraq was probably seen as an easy target. (Why would the US have massed its troops within Scud range of Iraq for months before the invasion if they believed Sadaam could attack them?) It seems likely that they thought they could find enough weapons to justify their invasion, but not likely that it was ever a reason for war.
* They really believed Hussein had ties with al Qaeda: Ditto: they knew that he didn't have any such ties and that Sadaam and bin Laden were fierce enemies. In addition, planning for the attack started in the first National Security Council meeting of Bush's presidency, nine months before 9/11. The Bush presidency showed no interest in terrorism before 9/11.
* Bush's floundering presidency: Like Thatcher in the Falklands, Bush used the war to win a second term, but I'm not cynical enough to believe that Republicans would kill hundreds of thousands of people to win an election.
* They wanted to finish the Gulf war: Embittered Republicans felt Bush.1 had been to soft when he didn't invade Baghdad and they wanted to finish the job. Or: Bush.2 wanted to one-up his dad. Or... various other explanations of the father-son relationship. Sounds unlikely. Most indications are that Bush policy does not emanate from Bush, but from his advisors. And the advisors surely can't be stupid enough to want to fight a war that had been over for more than 10 years.
* US neoconservatives wanted to start a new world order with the US as the global dominator, and they needed to attack someone as a show of force: This is Gwynne Dyer's main thesis in his book Future: Tense, and his arguments are persuasive. It also fits with why they decided to break with the Geneva Convention (the US is showing itself to be above all that on grounds of moral necessity). Still, I find it too cynical to be credible.
* They wanted to undermine the UN: This is related to the last item - they want to supplant the international power of the UN. This is true, although again it seems more of a side-effect or means rather than a motive.
* Iraq was a threat to Israel: It is true that several of Bush's key advisors had close ties to the Likud Party. But again, the lack of immediate threat and the toothlessness of Sadaam's regime make any argument based on threat to seem unlikely.
* They wanted to control Middle East oil: There is no reason to worry about oil supplies anymore. Even Iran has never held back oil sales from whoever wants to buy it. There was a theory that the US needed to invade Iraq in order to build an oil pipeline, but it has been discredited. Instability of oil supply was more of a Cold War issue, and could resurface if the US starts a cold war with China, but it isn't an issue now.
* Saddam Hussein refused to allow WMD inspectors to work in Iraq: Not true. He was very cooperative and they were able to do a thorough job.
* Sadaam threatened the US or US interests: Just the opposite. Hussein was quite cordial towards the US. Sadaam even consulted the US amabassador to Iraq before invading Kuwait, and appeared to get the green light to go ahead with the plan. (This was revealed in the New Yorker shortly after the Gulf War.)
* They wanted to make the Bush oil industry and military contractor base rich: That seems to have been a motivation in the way the war was planned, but it's hard to believe that even this crew would go to war for that reason.
* They wanted to create a permanent military base in Iraq so troops could be withdrawn from Saudi Arabia: It's true that the military presence in Saudi Arabia has caused some instability in the country, but the US could move more forces to Bahrain or Qatar.
* Humanitarian reasons: Bush? Ha, ha. Also, while I don't condone Sadaam in the least, the Americans have immensely overstated Sadaam's atrocities. The nerve gas deaths for which he was executed were part of the Iraq-Iran war. Most of the mass graves that have been found in Iraq were due to a 1993 uprising against him. Sadaam was no Pol Pot, not even a Milocevic.
To me, it just doesn't add up. You might say that it's difficult to see why anyone would start a war, but actually I can think of motivation for most wars, even if they're evil or irrational. You might also say that it's hard to see any reason for starting a war after it has been lost, and that might be my problem. Except I'm not sure that they ever cared about winning this war. More on that later.