At the time of the War there was a great deal of concern about the long term stability of the Saudi regime. I think the equations were very simple:
1. The US depends on foreign oil, much of it from the Middle East.
2. The only major Middle Eastern oil-rich country favorable to the US is Saudi Arabia.
3. If Saudi Arabia falls, it will fall to radical muslims who will be deeply hostile to the US and will use their oil production to exert unacceptable leverage on the US and the West.

What to do? Iraq is militarily weak, governed by a lunatic, and rich in oil. Therefore, take Iraq as an insurance against losing Saudi Arabia. Obviously other potential advantages would accrue in terms of controlling the gulf seaways and being in a position to intimidate other nearby states but I am convinced that the risk of losing unimpeded access to the Saudi oil fields was the motivation.

Is this acceptable behavior? Actually it is. It is the job of the government to protect the interests of the nation. Without oil the US fails (as do many other industrial states) and therefore protecting the supply by acting preemptively against the fall of the House of Saud is a rationale action of government. Is it moral? No, but the government is not paid to be moral, it is paid to be effective. Ask Mr Putin.