What are the moviations/reasons for invading Iraq?
Reasons such as gaining wealth from Iraqi oil, gaining security by controling a large oil fields by controling the country (continous occupation with maybe puppet type leadership). Or even because we want to scare other nation by miltarily dominating some nation. Are weak and mostly irrational views. They are of the same level of thinking, as imagining Aliens from outer space are controlling the nations of the world. Or that the "Face" on Mars is proof of Martian civilization. Or whatever.
Now, there is some truth to these idea. Iraq is important because it has huge oil reserves. And we would like to see a govt in Iraq in which we have good relations with.
Now, what are the moviations/reasons for invading Iraq?

I would say that 911 was a political turning point. It similar to the possiblity that the Columbia accident could be a political turning point for NASA.
Or historically it's like Pearl Harbor. Pearl Harbor is a "very good reason" for the United States to enter WWII. It proved the fact that we could just sit on the sidelines, and hope for the best. Of course, we were already involved in this war before Pearl Harbor- an isolationist would have to be pretty stupid to think we as a nation could actually be completely isolated [or be willing to do very extreme things, like sink every ocean going ship, cut international communication, and seal our borders]. But the moderate isolationist view was shown to be wrong. The mere potential of us interfering in a war was a threat, and of course, if you aren't a threat then you can become a target- as in the case with China and Japan (China was the archetype in an isolationist country).
Anyways 911 proved our isolationist type policies towards terrorism, wasn't going to work. So the question becomes how we stop treating terrorism as not our problem, but instead, how can we get involved so that we can resolve this problem. All Arab countries will tell you that the source of the problem is the conflict between Israel and Palestine.
We have been trying to solve this problem for decades. What is stopping this from being resolved? Terrorists. Who is funding these terrorists?
Step back a moment. Why was the US involved in the conflict between Ireland and Northern Ireland? Why wasn't this only a matter between England and Ireland? The main reason we were involved was because American citizens were supporting these terrorists. Similarly, Americans are also funding both sides of Israeli and Palestinians terrorists, but there is also Arabs (and Arab states) who are funding Arab terrorists. In order to solve the problem the parties involved with funding must come together and form an agreement- so as to evenally end the cycle of violence.
So this is and was the path we were on. But then there the terror groups who are at war with "western culture" and/or Anti-Americanism.
Since America was founded, democracies have been ever increasing. America was against colonial powers. America wanted a world in which all countries were democracies. Loosely, this could be called globalism. A central feature is world trade- free trade. Free trade means lot's of things, many people are against the idea because of the corruption which could be involved. Some are against it because it could bring a certain amount chaos in the world- it's dangerous. Or because people will lose their work, or whatever. The main factor is it will change the world.
At it's heart, Free Trade is a liberal idea- it's a desire to change the world. It's a very old idea, it's also new. You could say it's a collision between liberal arts and business school. It's international business 101.
These terror groups which are anti-american as a rule do not want this change, what they want is to return to some unattainable golden time in the past. It's a rather *bizarre* because the Middle East was the center of trade in the old world, it was the land route where East met West- The Middle East was and still is "all about trade".
Anyways this is getting boring.
The thing is if you want to get involved in solving terrorism which is anti-americanism. What do you do? This anti-americanism terror, was getting support by the endless conflict with Israel & Palestine. The song was America was causing this conflict.
The other point was that America was occupying Saudi Arabian- wrong again but you can make up lies about it. Iraq was at the center of this problem. Iraq simply would not work with other countries. It was worse than Iran in this regard. Iraq was supporting sucidical bombers. Iraq was the reason we were in Saudi Arabia. Iraq was at war with America. Iraq country of terror to it's citizens. Iraq was a mad dog in the region, caging it wasn't working [it hurt the region, Saddam was using it as club on his own people, and nations [France] were undermining the policy], curing it was impossible, if you wanted in be involved [rather isolationist] the only solution was to kill it.