Keeping it short:
A lot of people say we're only in Iraq and Libya and so on for the oil. Then some people counter by saying that the majority of our oil comes from Canada, then Mexico, then Saudi Arabia. Well, I want to counter counter that when a country uses US dollars to sell oil that means that if any country buys oil from that country, they have to sell something to the US Federal Reserve, in return the Fed gives them money, then that country buys the oil, THEN the country that sold the oil invests their money into the US economy.
Which explains why we get pissed off when people like Qaddafi want to sell their oil with gold, rather than US dollars.