The war in Iraq was over oil. Where people were mistaken was that they believed it was a war to make oil cheaper. Have you noticed that since the war in Iraq, the price of oil has gone up?
The problem was, Saddam was flooding the global market with cheap oil, oil that the US had no stake in. This meant that all the oil over which the US had a controlling interest, particulalry that from North America, Sth America and countries in the Middle East with which the US had good trading relations (like Saudi Arabia), wasn't being bought. People prefered Iraq's cheap oil over the US's expensive oil. Solution: invade Iraq, take control of the oil, or at least cause chaos, stop the oil from leaving Iraq and thus force people to buy expensive US oil