Consider two things:
1 The US is by far the largest user of oil.
Our own reserves, combined with Venezuela and Mexico could not sustain us. We could not meet demand when OPEC cut us off in the 70s, and we use far more today than we did then.
2 Iraq has around 25% of the proven oil reserves in the world. The Persian Gulf has 50% all together.

No, oil was not the only factor. Partly the war was an excuse to increase military spending, to set a precedent of ignoring the UN, and to generally get citizen support of increased government - particularly executive - power over both our own lives and the world.
Partly it is a warning to other countries that the US is not to be messed with, that we should be given whatever we want without having to ask twice.

Basically it is about world domination.
However, oil is a huge component of that, and it is not coincidence that we choose as our target the country with the second most oil reserves on the planet.
The are plenty of other countries with human rights abuses, genocide, a lack of democracy, or Islamic based governments.
We did/do not send massive amounts of troops into Rwanda or Darfur even though what happened/is happening in those countries is at least as bad as anything Saddam ever did.
Saudi Arabia has a King and no Parliament or congress. There are no elections in Saudi Arabia. There never have been. Saudi Arabia is also a Islamist country in which law is based on the Koran. People have very little freedom.
Yet, Saudi Arabia is our ally.
Iraq, by contrast, was a secular government. People had far more freedom in Iraq than in Saudi Arabia, and in fact, more freedom under Saddam than they do now (for example, in most areas not directly controlled by US troops today women must wear head scarves).

Ultimately it is about preserving the American way of life - i.e. CONSUMPTION. Massive, excessive amounts of consumption.
When our leaders use the term "democracy" what they actually mean is "capitalism". Unrestricted free trade is what allows our corporations (which are becoming more and more synonymous with government) to become as excessively wealthy as they are.
If we did not display a giant show of force now and then, the rest of the world would not put up with us. Iraq is a warning to OPEC. We leave Saudi Arabia alone because they give us a fair price. Most people, when making a deal with the mafia or any other bully, offer a good price, and then except what ever terms are suggested. Saddam had balls, so we cut them off to serve as a warning to everyone else. Getting control over the immense Iraqi oil fields is much more than just a side benefit.

It is about power. We are exploiting the world. This is why illegal immigrants want so badly to get in here. Everyone would prefer to be on this side of the fence, since we have both the guns and the money.

The "threat" of "terrorism" is no different than the "threat" of "communism" a few decades ago. What did the communists want which made them evil?
They want to take away the money of the wealthy, and spread it around.
That's it.
Nothing to do with democracy or authoritarianism. It has to do with capitalism. Russia was a democracy. Not a perfect one, but ours never has been either (look at a Gerrymandering district map).
And they were able to convince the American people that it was an issue of good (us) vs evil (them).
Even today you see people on this very form using "Marxist" as an insult, with no context or basis. And so it continues...